I don't know if either of these are mentalism, but they both probably relate to the unconditioned purity of a young mind.
1) when my daughter was about 4, she amazed us by correctly identifying the cards we were holding, without fail. She was so cool and relaxed, she just looked into our eyes and calmly predicted the cards one by one as we held them in front of us.After a furore of excitement, wondering what to do about this phenomenon we eventually realized she was looking at the reflection in our eyes! Something which failed to do when we tried it ourselves. She had never been shown this technique.
2) a few years later, I was playing rock, paper, scissors with the same daughter, and a couple of times I correctly had a 'run' of about 10 or so wins in a row. To be honest there were probably a couple of more random results mixed in but I particularly remember these 'runs'. The odds of this are pretty staggering, but I think I must have tuned into her mental processes. (Right, I'll try that one again he won't expect that, or instead of one before, i'll go the one before that etc.). Needless to say, it didn't last and we haven't played it much since, I think I recall she beat me last time we played!