These aren't browser games, but could still be fun.
I used to play Battle for Wesnoth a lot as a kid, both with my dad and by myself, and as it's turn-based it should run fine on a raspberry pi.
OpenTTD could be fun, although it may be a little complicated depending on how old they are. It's not very difficult in terms of challenge though, it's more of a sandbox once you get past a certain point. There's also sandbox options to give yourself lots of money if you didn't want to worry about money and just build cool trains and stuff.
I used to play a lot of Supreme Commander with my dad as well, so maybe something similar like Beyond All Reason or ZeroK could work if they take an interest to that kind of game. I haven't tried them though.