Washington state camping

Bay view state park is close. It is a nice park.

Further away is lake Chelan.

Have you looked at HipCamp? It's sort of like AirBnB but for camping. A random search of Whidbey RV camps brought this up near Deception Pass: https://northwhidbeyrvpark.com/

We use Hipcamp a lot and really like it. One of the places we have stayed is on Whidbey on the southern end near Greenbank and is called Fern Gully Camp. There are only 4 spots. Another option might be Fort Casey, have you looked at that?

Maybe one of the San Juan Islands, but I have a feeling they are booked and perhaps that might be too far for them.

The region has been experiencing a tourism boom for many years now, and the days of just "winging it" in high season and finding nice facilities are but a dim memory (and a weekend in July-August is the definition of high season here). So as April is about to break, it's late in the game to be booking campgrounds (and lots of other things) for the summer season. Many places get immediately booked up within hours (minutes?) of when they open up for advance reservations. You may also find that there are surprisingly few public campgrounds that have the sought-after amenities like showers. You will want to cast a wide net and be prepared to make tradeoffs. If you're willing to drive east up into the mountains, or take a ferry across the sound to some of the less popular spots around the peninsula, you can probably find places, but even in obscure spots with few amenities, you may find places fill up quickly/early.

What David said is quite true for State Parks and National Park/National Forest campgrounds. BUT sometimes you can find nice parks through County sites. Try your luck

https://snohomishcountywa.gov/facilities/facility/details/kayak-point-regional-county-park-51

I do like Bayview SP a lot. They are probably booked up, but maybe not. Camano island SP is also nice - too bad Cama Beach State Park is closing (or closed) - it was one of our family favorites.

I can find camping by Port Townsend, but that is a relatively long drive for them.

I am surprised 50 miles is a " relatively long drive for them."

Deception Pass is ~80 miles, so why is Port Townsend seemingly 'too far' ?

From their home, Whidbey island takes a bit over an hour. Port Townsend is a bit over two, if the ferry is used. The last I heard the ferries between Seattle and some of the islands were problematic, but I live halfway across the country. We have been to the Olympic peninsula and close at least three times and that drive down I-5, etc., always takes much longer than what google calculates.

My calculations are as of right now (8:41pm, Pacific, on Friday) so likely there's less traffic than other times of day. They can only spend the weekend with us so the travel time becomes more important. Also, I would tend to agree that the travel time difference is not all that significant, but its been quite some time since I've traveled with toddlers.

I looked at Kayak county park, and it looks awesome and we'll keep in mind for the future. Its fully booked for our dates. And Bayview looks nice, too, and fully booked.

I did take a look at Hipcamp and I have a couple places on Whidbey that look as though they'd work well.

It sounds like the whole crew wants to do Whidbey Island July of 2026. I guess I will be booking Deception Pass the minute reservations open up!

I knew I'd get some help, here, thanks all. Now back to planning for my trip to Europe!

No recommendations (sorry) but I learned the hard way this year that WA state park camping reservations open 9 months ahead, not 6 months like Oregon state parks and other federal sites. You'll want to be online close to 12:00 a.m. the morning your window opens. Sad reality of camping in the PNW, especially if you want an RV hookup site in a popular place.

One of my favorites is Washington Park, which is operated by the City of Anacortes. They have some absolutely massive sites and a good mix of tents and RVs. Anacortes is an OK town, but you could also walk on the ferry to San Juan Island. The ferry dock is in downtown Friday Harbor, which is cute. They have had a shuttle that will take you to Lime Kiln Park, where you can often see orcas. I don't know if that's still on. https://www.anacorteswa.gov/560/Washington-Park
Rasar State Park is near Concrete, which doesn't have a lot going on but we did get good ice cream there. I have only stayed in the group camp there, and access to the river is a bit of a schlep. Nearby Lake Shannon has a boat launch and a small park. Baker Lake is probably not out of the question as an outing from Rasar. There are Forest Service campgrounds on the lake.
The ferry to Port Townsend is pretty small, so with a camper, you'd want to reserve as soon as possible (or drive around). For the San Juans, that means two months ahead, 7am. I assume it's the same for all routes.
Speaking of the San Juans, I wouldn't even try, but the county does operate several campgrounds (Odlin on Lopez is the one I am most familiar with). In addition to the state parks, some of the campgrounds (on SJI) are operated by the feds, I think. Private camps on Orcas (Doe Bay Resort), Lopez (I don't remember the name) and SJI (Lakedale?), maybe others. Travel time to the San Juans is LONG.
If you're willing to travel to the San Juans, take a look at Silver Lake, which is a Whatcom County park reasonably close to Mt. Baker (which might even have snow on the ground still, but certainly has great hiking and views). Boat rentals, some large sites. It's been awhile since I camped there, but it's nice.
Birch Bay State Park has a massive (rocky) beach that would be great for little ones, probably full but worth checking. There's probably a private campground up there, too.
Have you tried the Forest Service campgrounds? I find they have larger sites (thus, more privacy) than the state parks. And if you have (or care to obtain) a membership with Thousand Trails, they have some nice places in our area (with which I am not at all familiar).
If you find a spot and have questions about the park, you're welcome to send me a private message. I haven't been to all of them, but I've been to a LOT of campgrounds in NW Washington.

I will tell you about a little known resource for some great campgrounds . Clallam County Parks. There are two great campgrounds that take reservations and most people, except locals don’t know about them.

One is outside Sequim and one is past Port Angeles. Take the Edmonds -Kingston ferry. You will arrive at the Dungeness National Wildlufe refuge/Clallam county park about an hour later. Lots of great opportunities nearby.

Is your son and his family actually IN Seattle or just nearby? Knowing what town they live in would help since some places are just too far for visiting each other on a trip like this.

My son and his family live in Seattle, north of downtown in Greenwood,

I know Greenwood well, as both our adult kids lived there for a while.

I would not attempt to go anywhere on the Olympic Peninsula or even Port Townsend as those are too far for frequent visiting. I would stick to Anacortes/Whidbey. Also Flowing Lake in Snohomish is a possibility.

From their home, Whidbey island takes a bit over an hour. Port Townsend is a bit over two, if the ferry is used.

From their area of Seattle "Whidbey Island" being only a bit over and hour away would mean taking the Mukilteo -Clinton ferry. Clinton is at the south end of the island. Decepton Pass State Park is at the north end , almost 50 miles from the ferry. There is a big difference of just being on the island and being at the State Park.

The alternative is an ~80 mile drive going N or I-5.

From their area of Seattle the easiest/fastest way to Port Townsend (PT) is to take the Edmonds-Kingston ferry and drive. The only way to drive all the way is to go south through Tacoma and then north past Bremerton. This is a 125 mile drive that google lists at 2 hr 20 minutes. ( if you make it that fast, buy a lotto ticket, because it's your lucky day)

Thus my puzzlement about the earlier comment of PT being to far. PT is actually fewer driving miles than Deception Pass, but a bit longer trip due to the ferry.

The last I heard the ferries between Seattle and some of the islands were problematic.

Generally 'the islands' means the San Juan Islands which are accessed from Anacortes. None of your projected travels would involve those ferries.

Happy planning