Hi Malca - I'm not sure if this is quite what you're looking for but you might want to try
MorphVOX. It's a voice changing software and one of the included profiles is that of a child. It sounds authentic for me (it adjusts the pitch etc for you but you can also tweak it further). Hope this helps. :)
aT
Oh, I forgot to add that it has a fully functional 7-day trial. :)