I 2nd that.
If you are going to the dollar store, stock up on band aids, toothbrushes, first aid ointment, etc. Although the gesture of gift giving is very kind, supplies that are hard to come buy or expensive for the average Mexican, are very very much appreciated.
Ask around, it won’t be too hard to find a struggling neighborhood, just make sure your not wandering in to a bad situation. Locals, and collectivo’s normally can help you find a family, or school that is in need of such supplies.
Another way to help out without lugging down so much stuff, is to just shop with the locals. By local produce, if you don’t want to consume it, buy it and ask a collectivo to take you to a family that is in need.
There are many many ways to show your generosity.