The pattern should have some information at the start which says "X number of stitches to 1 inch; Y number of rows to 1 inch". I would try doubling your yarn and knitting a test square to see how close it is to the specified gauge.

I imagine it is same as using sport yarn for crocheting, use smaller hook & animal will be little smaller. So you would go down as size or 2 with your knitting needles & have a little smaller animal is all. Since it is not an article of clothing it doesn't matter if it's a little smaller unless it would be so small the child could choke on it.. Worsted yard is usually 4 ply. Sport is usually 3 baby yarn is 2. So that gives you idea of the different main yarns used. Lot of specialty yarns now & chunky, ribbon etc. If it's actually baby yarn than colleen is right to use 2 strands of it as it could end up with a lot of holes if yarn is too thin. Stuffing would pull out easy then.

Colleen, no, the pattern does not specify guage. I will take Sunny's suggestion to use smaller size needles and expect a smaller bunny.