Baby mum it's mostly a logic thing in the beginning. Most often thumb is on c. Do check out softmozart s forum as there are many videos that show correct fingering.
http://softmozart.com/forum/55-teaching-videos.htmlCheck out nursery one album on the link.
Once you watch a few you will have a pretty good idea. Some piano teachers are really pedantic about getting the right fingers on the right keys but from my experience with my kids the ones who were given more freedom stress less about hitting wrong notes and so are progressing faster. One of my girls struggles to move from chord to chord or position to position because of the emphasis on correct fingers when she first started playing. It's like her fingers are glued in one place. Allow for some creative interpretation but just encourage multiple finger use is my advice.