I wanted to do something similar! I messaged my local library with a small proposal, but they never got back to me, very disappointing.
Suggestion: Talk to the people face-to-face, and have a demo ready. If you tell them you want to do something for kids, in the demo there are free cookies, and besides offering the space they don't have to do anything, then the chance is almost 100% that they jump in.
PS: Some more traditional organizations don't even read their emails at all. So don't take it personally.