Well, I wouldn't say it isn't for kids - especially as one of the photos I recently entered was taken by my daughter - but I guess it is aimed at the posting members - not that your children can't enter of course.
It is always incredibly hard to judge, and to be honest, I don't, I tend to let my children choose their favourite as I can never choose a favourite. I think with digital cameras becoming better and better, the photos are going to keep getting more and more amazing.
Just so your little one doesn't get too disheartened, my photos often go unvoted.