Both the library and the elementary schools are in need of basic supplies - paper, pencils, glue sticks, crayons, colored pencils. Last summer the library was especially interested in children's books in Spanish. I haven't spoken with anyone in a few months so I don't know if those are still needed, but my hunch is they are.
It is worth contacting Djean for a list of specific items needed by the school in Dangriga. That city is not visited by as many as San Pedro, thus the need is greater. You can leave a box of supplies at the Maya Island Air desk when you land in Belize City and they will forward it to her in Dangriga.
As far as bringing things just to give away to the kids on the street - I recommend an older post by Reef Ransom on this subject. A search on this board for "gifts" should bring it up.