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).
Pictures
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.