Travel daypack for kids/tweens

https://store.ricksteves.com/shop/p/civita-day-pack

I have several Rick Steves bags and they are well worth what you pay for them. They hold more than you think, and they are very lightweight and durable. The Appenzell is the one I carry as my airplane bag (fits my laptop) but it's too big for what you need. I think the Civita would be perfect.

The one I have is too expensive for three kids (155), but it does check all of the boxes (Fjallraven Kanken Outlong), and the bottom separate compartment has made it invaluable to me. I am sure something comparable can be found. Have you checked an outdoor outfitter?--it might take some in-person searching.
Zappos can be great for searches like these.
This LL Bean might fit the bill: https://www.llbean.com/llb/shop/129987?page=llbean-stowaway-tote-pack&bc=29-915-916&feat=916-GN0&csp=a&attrValue_0=22&pos=26

I got this one from Costco for my 15 year old grandson for our upcoming trip to London and Paris, and it's really a nice backpack. He loves it so far. https://www.costco.com/lol-quilted-laptop-backpack.product.4000295754.html

You also might check LL Bean especially for the younger kid (I got one for my 11 year old granddaughter). They have a wide variety of really nice and durable backpacks in fun colors for kids. https://www.llbean.com/llb/shop/818?page=school-backpacks&csp=f&bc=50-816&start=1&viewCount=48&nav=ln-816

Celeste, I am a yearslong devotee of the RS Veloce bag (I just bought the new one because of the luggage pass-through, but my old one is still great; I use it for travel and work), and was sleeping on the Civita. That really might be the ticket. (And certainly fits the budget!)

I'll search the forums for water bottle capability; certainly someone has commented somewhere on that! Thanks again.

You might want to consider one of these: https://vapur.us/collections/anti-bottles
I stopped carrying a hard-sided water bottle. They're heavy and take up way too much room. The collapsible ones are much more practical. Helpful hint for washing it - put a chopstick in it to keep the sides separated so it'll dry thoroughly.

For me the Eddie Bauer 20L stowaway backpack would be perfect; make sure to catch it when it's on sale.

My kids always had LLBean backpacks, great warranty, however the packs were so durable that we never needed to replace.

Take them to REI. Bring the (filled) water bottle, iPad, etc., and see what works. REI sells kid-size backpacks, which may be too small for the 12-year-old, but there should be a good range of smaller daypacks in adult sizes. If their picks are not REI-branded, you can look for them on Sierra.com, where the prices are generally lower. Don't forget to check the used section at REI, as many of those items really aren't used at all.

The K-12 Katpack Backpack by Red Oxx is perfect for what you need. It comes in 12 different colors so they can each have their favorite color and no arguing over whose it is. Also, they have a lifetime warranty and will repair or replace at no charge. I have bought 3 of these (2 for grandchildren and 1 for myself). When I commuted to work via bus for approximately 50 miles each way, that is what I used to carry my small laptop, lunch, breakfast, and personal items in every day. I now work fully remotely so mine only gets used as a underseat bag or in the car for road trip. Here is the website: www.redoxx.com. They are expensive but when you consider that they will last a lifetime and the kids can use them for school without wearing out. Good luck in your search.

Lots of good advice here. Check out Patagonia.com. Some colors on sale. Kid’s refugito 18 liter and atom 20 liter tote pack. Check out Osprey.com.

I put beverage bottles with screw on lids directly in main compartment of backpacks.
Lots of choices in this category.

Well I always carry a pair of pajamas for my kid, when I travel. It is super comfy & looks elegant. Do you guys wear pajamas on our trips? Let me know how do you feel?

Tonya - The bag sale just dropped! Makes that Civita an even better deal - only $20!
https://store.ricksteves.com/shop/p/civita-day-pack

I travel with my kids a lot and one top tip is to try to have a clip of some sort (caribener to loop or zipper pull) to secure the water bottle to the bag. The bottles tend to pop out when the bags are under seats or generally moved about. And we never take favorite water bottles anymore, something cheap and cheerful and lightweight fits the bill.

As for backpack anything the kids are comfortable with will work. My son likes very minimal and my daughter likes something a bit more structured with pockets to stay organized. The RS options are good as are Eddie Bauer. I also have successfully used cheap Amazon packable day packs.

At that age chargers are a big issue. (And still are at 14/15!) I have names/washi tape on everyone's and they are responsible for keeping track of their own. (Learned the hard way that their more responsible younger sibling does not "owe" them buddy breathing. 😂)

Check out the Osprey Daylite plus. Great pack and they come in many different colors so each kid can pick their own uniques pack. Always was an issue for our girls.

My husband and I use these daybags. They come in 3 sizes and pack up super small. The smallest size is 16 L.
https://www.amazon.com/dp/B08BRB2BSD?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1