What sort of gifts should I bring for children in Madagascar?
Gifts for kids are tricky. Most people say you shouldn't give things directly to children, rather gifts should be directed towards responsible parties in a community -- parents or teachers -- on behalf of the kids. Giving things directly to children can encourage begging. Pens/pencils and nutritional items are good. Stickers and balloons are enjoyed but not particularly useful, and candy can cause many problems.
If you have the time, a nice thing to bring is a handout with English and Malagasy words on it so kids can learn a few words and phrases. There are a couple of resources at https://wildmadagascar.org/media/ and https://wildmadagascar.org/people/malagasy-english.html which might be of interest for distributing. I personally like the one with the wildlife in English and Malagasy (at the bottom of the list).
If you take pictures of kids and tell them you will send them a photo, you should honor your word. Even if it takes months for a child to receive a photograph, it will still be a very special moment for him or her. It may be the first photo the child will ever seen of him or herself.
For the truly dedicated techies, bringing a digital camera printer could make the photo giving process instantaneous. Be wary though of creating unfair expectations for future visitors. It may be best to delay a bit before giving pictures -- perhaps wait over night if possible. Also power can be a luxury in Madagascar so be prepared with plenty of batteries.