Don't make it a business transaction. She helped you because she loves you and wants to strengthen the relationship. That is her reward. So show her that reward, by showing her that she is closer to her little nephew. When kids are that young you need to spend time with them frequently in order to develop a relationship, and so having her visit and help out was a way for her to do that.
That's why I like the idea of a photo album, or pictures that your son has drawn. It's not a thank you per say, it's more of a, "You put in some hard work, and now your nephew knows you better."