Personally, I wouldn't buy them any other toy sets. You've got a lot of good systems. For my ds what brings those to life is having stuff to use WITH them. Like I went to Dollar Tree and got him tons of supplies. You can get
Just walk through there and think creatively. They have little hack saws (dangerous obviously, don't do that), all sorts of stuff. My ds LOVES their colored rope stuff. He loves the twine. The duct tape is kinda cheap. Masking tape is good. So he'll tape blocks together to create things and use rope. There's synergy with more supplies to use what he already has, kwim? Then it's all in sort of a maker bin.
Also consider investing in figures they can use with their blocks and systems. Playmobil people were HUGE for my ds. He has scads of them. He likes the intricate details, and they work well with blocks, etc. You can buy them in the little bags for $3 each, so instead of investing in another system, get them each like 10 or 15 people, kwim? Way more fun. We have lego people too, but the playmobil ones are bigger and nicer, I think. Or go army figures or whatever they're into. My ds is crazy into army figures now, so he combines all the pieces he owns from all these systems and builds the tanks, recreates the wars. It's pretty cute.
Also consider pictorial encyclopedias in their areas of interest to drive their designs. They might like books of bridges, books of airplanes, etc. DK/Smithsonian is a place to start, then in a few years they can get more specific.
Btw, have you seen the K'nex education sets? You can pair the bridges set with books on bridges. None of that's independent though.
You might try a class or a maker camp to inspire them. There is an org that sponsors maker camps during the summer. You could see if they have them in your area. Your 6 yo would be old enough maybe.
Edited by OhElizabeth, 14 November 2017 - 12:29 AM.