I think that Rikita seems to have done enough travelling and camping and such that she should be able to manage just with a backpack. Agnes could have a little light pack with her coloured pencils, a book and maybe her pyjamas or something light. That way she will feel like she is helping and a big girl.
If you choose to camp and do a bit of hiking (or going for walks in this case), then dragging a case on wheels is really not practical. Better to limit what you take than load yourself down with even more stuff.
well, the backpack was heavier than planned, but i managed okay - and there wasn't much i could have left at home. maybe one t-shirt, one long-sleeved shirt and one pair of light trousers less for each a. and me, but then, i was happy to have them when i got very wet in the rain just after having washed some of the other clothes (which thus were wet, too) - and i by mistake took one small empty silicone bottle for lotions and one tooth paste too many - but altogether, that'd have saved me less than a kilo ... but looking at all the things other people with cars had at the camping places (apart from huge tents), i think we managed pretty well with just the essentials ...
In general, I think people tend to overpack, especially if they have a car because they know they don't have to carry it. I learned by experience to pack light and have taught my childrent to do so as well. Although, when my daughter was 14 and went to the Alps for 2 weeks in the summer, when the kids went skiing on glaciers, she had to put socks on her hands because I hadn't thought she would need gloves!
And on my second long trip abroad, I went with 2 friends. We landed in London with a ton of stuff, since we were leaving for 3 or 4 months. By the time we got to Amsterdam, Iwent to a post office and sent home almost everything. I bought a small leather bag to use for the rest of the time. And I managed although without buying any souvenirs. The two friends kept their suitcases (not even rolling ones in those days!). One girl went off to Israel to go to a kibbutz. As she was hitchhiking on the side of a road, the guy who stopped to pick her up drove over the suitcase by accident!