unplugged scaffolding
Because the birthday of one of the children is comming up, I've searched and found a few books and games that you might like. Usually I give something for their birthday that is a developmental challenge for the child at that time.
The books are in Dutch but the games might be interesting. I'm going to give a link to a dutch site and try to find an English counterpart. Thinkfun, Dutch site that delivers pretty good games and a site that describes abstract games are among my favorite. Wooden puzzle challenges can be difficult even for very bright children. That's good!
I love difficult.