Yes, it seems that smaller creatures can get into the valley quite easily. But that doesn’t explain how the grown up leaf eaters can come and go as they please, but the grown up sharpteeth can’t. If I remember correctly one of the TV episodes kind of explained this because there is a place where a lot of fruit grows that sharpteeth absolutely HATE the smell of. It helps keep them away, but leaf eaters like the smell and taste of the fruit. When Littlefoot & co ate too many of the fruits, Red Claw almost got into the valley. I suppose all the herds and such could come through there.