Don't just give them the kit (any kit) and leave it at that. Sit with them and YOU start making stuff with them looking and get them more involved as you go along.
I remember my first geek activity. It was when I was 4-5 years old . My father took apart an antique clock apart and cleaned it and later asking me to do small things to do. That is what got me hooked to DIY. Later when I was about 7-8 years old my mom used to read short Sci-Fi stories to me and that's what got me hooked into reading.
Spend time with your kids, they will be far more involved with whatever you do.
As for the kits, anything from Chemistry kits to making models are equally simulating the important thing is getting started at an early age.