I can suggest a forum...check out turtleforum.com
But I can tell you right now that everyone there is going to suggest that taking an animal from the wild is NOT a good idea. And I might add that children need to be highly supervised around any wild animal, but in this case its more a concern for possible disease and parasite issues. Until you decide what to do, do not let the kids handle him/her, and if they do come in contact, wash their hands very thoroughly.
I am more familiar with box turtles and tortoises, so I can't help much with the care of a water turtle. But based on my sisters water turtles, I can tell you that a 55 gallon tank is a minimum size for one of these, and you will NEED a respectable filter!