I'm on the iPad so can't upload images but I'd check out the concept art for Disneys Brother Bear, they have wonderful shapes for the bears! I'd keep playing with the characters in different positions and situations to really help bring out their characters, by repetition you'll refine the designs. Remember the three basic design shapes, a circle shows easy going, friendly characters, a square shows stern, stubborn characters and triangles show energetic, edgy characters. Keep up the good work! 
I think you are headed in the right direction. I like that you are distinguishing your characters with different hair shapes. To me, they do feel like kids, and they have an appealing look. I think you could exaggerate even more, but that's just a personal preference. I do think they work as is. If I'm interpreting this correctly there will be a bear version and a human version of the same character Callisto, and the same for Arcas? If so, I wonder if there is a way to make the bear version and the human version visually tied together in some way. With color for example, or perhaps proportion. Just an idea. I also think it's ok to have a muscular kid, especially if it's someone like Zeus. . . though it could be interesting if he was terribly skinny as a kid. Anyway, just throwing out ideas that you can feel free to disregard.
One thing that I think could be improved upon is how the older bear's head relates to the body. It looks like it's sort of cut and pasted on at this point, and I think it could transition into the body better. I also added a little nub tail, but I know that the tail is a part of the myth, so I'm not sure what your plans for the tail are.
I would also be aware of how you pose the pegasus. The back legs feel a bit static for the pose.
Good luck with your next stage! It seems like a fun project.