Found these, but you will have to investigate further to see if they are suitable for little ones:
http://free-fashion-coach.blogspot.c...-download.html
http://tifaia.deviantart.com/art/Sem...tern-289765104
If you have not sewn really stretchy swim-type fabric before, be sure to Google tutorials on working with it.
Also found this Youtube video, but I haven't watched it so don't know what they use for a pattern:
http://www.youtube.com/watch?v=jRKEEz1d2Fg
Edit: This one might be closest to what you are looking for:
http://www.patternschool.com/?page_id=197