maybe ask for some donations so you could get some stuff from the hardware store? and how old are the kids? teaching them how to build things would probably be pretty cool too, but i understand that theres a liaibility involved when you hand a kid a hammer or a drill. its really rewarding to build something and then skate it though, no matter how old you are.
if you cant get much money, id go around some industrial areas and look for pallets and scrap wood. i know a place near me that manufactures trusses for roofs and they leave a huge scrap big of good wood for anyone who wants to take it. also, if you find a place that does something like that, maybe try to talk to someone in charge and tell them what youre doing and see if they could set some stuff aside for you. ive had a few projects in college where i needed quite a bit of cardboard and someone from our class talked to this company that made boxes and they donated a shit ton of cardboard sheets to us as long as we came to their shop and picked it up. it never hurts to ask.