Can't help that much, because I haven't read all of them thoroughly
, but the PDFs I have include:
Bunnies and Burrows (but from what I remember, not exactly for kids and based on Watership Down);
http://en.wikipedia.org/wiki/Bunnies_%26_BurrowsCat RPG - which is just based on cats and I don't think has a D&D kinda feel in that way - but I think it's a free download somewhere;
Mouse Guard, which is just mice, heroic mice though
;
and Tales from the Wood RPG - which does have a variety of animals, it was hyped as the "woodland creatures RPG" IIRC.
Hope that helps a bit. I bet Tales from the Wood might be your best bet for kids.
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