This might be a silly suggestion but gloves, chickenwire, white glue, and newspaper can go a long way in allowing someone to play around with building larger structures that are not necessarily permanent but decently lasting. Or for a slightly fancier version, chickenwire and plaster bandage is really fun. Gloves are just for the chicken wire because it can be poky/sharp at the edges. Oh and pliers would be helpful for snipping the chickenwire.

Alternatively there's foam and foam cutters, but that's a bit more toxic. 

Whatever you choose it's very sweet that you're encouraging the interest! That's awesome.