6 weeks in Europe with young kids

Planned date of travel would help to give advice. May be I missed to getting it from your text?

And what is RV for you?

Mark raises good questions.

Just a quick note that if you decide on Denmark, I had a wonderful stay at Steel House, a hostel in Copenhagen. I see they have four-person rooms at around $267, still a lot admittedly, but I can imagine this hostel as a good place to stay with a family. The snack bar is always open, there is a kitchen, they have bike rental (don't remember specifically about accommodations for kids, but since the Danes have every imaginable contraption to attach to bicycles to carry kids and cargo I imagine they could make it work). There is a large living room kind of area for everyone to spread out, and there are laundry facilities (the only time I had to use coins in Denmark). The location is good. From Copenhagen it's still a trek to Aarhus by train, but should be a pretty ride. If you decided to stay in Copenhagen, there are great day trips, including the Louisiana Museum, an outdoor sculpture park by the sea with plenty of room for kids to run around the funny sculptures (although you would have to watch that they not fall over the cliff in a couple of places). Of course Tivoli Garden is another possibility for an amusement park quite different from what the children may have seen before.

Thanks - updated the original post. Mid-May through end of June.

The RV = a large van outfitted with sleeping arrangements and small kitchen and bathroom. My husband was looking at one on “indie camper.”

Thanks, Nancy - those are great recs for Copenhagen. We definitely want to see the Louisiana Museum!

Thank you for answering.

A good travel period in Europe. From weather perspective I would do Denmark more at the end (temperatures).

Be aware of different countries'

• Schengen member: yes / no
• currency: Euro or own
• holidays such as Pentecost or national holidays, e. g. Denmark June 5
• general conditions such as shops open / close on Sundays
• regulation, e. g. driving permission (not license only) or special car batches
• travel advice and / or warnings by authorities, e. g. UK's foreign travel advice
• level of speaking English. DeepL is a high-quality translation tool, also app.

Transport: flight, train, bus are good ideas. Omio app helps comparing also different modes of transport. Cross-border car rentals are an expensive idea. Wild camping is forbidden in most parts of Europe. RV or camper vans must "sleep" at camping locations - penalties can become expensive.

Accommodation: hostels, family hotels and budget chains are your friend. Airbnb and the people using housing spaces as short-term rental are minimum seen critical up to hated in most European top destinations. Avoid this if you want to travel social responsibly.

All countries are kid-friendly. Denmark is a very family-oriented country. Do not miss Legoland, Lego House and vikings at Roskilde (near Copenhagen). A funny photo in Aarhus can be the family looking through the "Sea Pink".

For best memories I recommend using a travel zoom camera with optical zoom 30+, not smartphones only.

M,C.E.
Sounds like a great trip! My only caveat...renting a vehicle in one country and dropping it in another can cost between 500-1,000 extra euros. It's not like dropping in a different state here in the U.S. Going from Spain to Portugal is not one of the best train options, unlike the rest of Europe. We found a flight was best. RVs are popular, but I would stick to one country for rental and drop-off. France could be a good option for that. They have lots of camping sites and it is popular there. I would also fly from Portugal to France. And maybe rework your France itinerary? There seems to be some backtracking there......French Alps, then Paris. Re Biarritz, I would research trains from Lisbon or Porto (a nice stop) to Biarritz, then Biarritz to wherever in the French Alps you would like, before Paris.
Do be sure to factor in that each travel day is pretty much lost to that travel. (Checking out of lodgings, getting to airport or train station or picking up rental vehicle, the actual drive/ride, orienting yourself in New City and getting to lodging, checking in.) You will of course be somewhat slowed down by the children and the attendant gear, like a stroller or car seat (NOT a criticism).
If you do drive at least part of the way, be sure the rental vehicle has the car seats for the little ones. I know France is especially strict about that. Of course, you already know this...just a reminder.
Have a wonderful adventure!

A few years ago, I rented a campervan in Munich and used it to visit Austria and Switzerland, your more expensive countries. It worked out great because all three countries had excellent, family-friendly campgrounds, many in close proximity to cities, which we reached via public transport (or foot, in one case). This might be a good strategy for visiting Denmark, a notoriously expensive country.
My recollection is that traveling by train from Spain to Portugal isn't as easy as one might expect. Here is the Man in Seat 61's information:
https://www.seat61.com/international-trains/trains-from-Madrid.htm#Spain-Portugal
Overnight trains (and ferries--e.g., from Italy to Spain) kill two birds (lodging and transportation) with one stone. The 5yoa child would probably enjoy the bunkbed experience.
Every move with kids is exhausting for everyone. Slow down and stay longer in fewer places.
Have you found a direct flight from Crete to Spain? Because I am not seeing anything. This is where I would start. Where can you get a direct flight from Crete? Connecting flights with kids are no fun. The same goes for the end of the trip. What cities have direct flights to your home airport (the answer for the NE USA is "most," in my experience). All the internal flights will be (relatively) short and inexpensive.

Price of fuel is currently around $10/gallon in Europe. ( data from Portugal and UK). Be sure to figure in fuel costs as part of the rental cost for the RV.