The World Wildlife Fund has "adopt an animal" gifts that would be perfect. Here's an example of the elephant, but they have many other animals available:
https://gifts.worldwildlife.org/gift-center/gifts/Species-Adoptions/African-Elephant.aspx
For $60 (which is a generous amount for a first communion), you get a stuffed animal, picture, certificate, etc. I like this idea for a small child, since it gives them something tangible to remember your gift.
Otherwise, money or non-religious gift are both fine. If you give a toy or book, I tend to choose one with lasting value--an illustrated book about animals rather than Captain Underpants, for example.