If a young child has hit his mouth, and there is lots of blood especially on his upper teeth lip area, look and see if it might just be a torn maxillary frenum. That's the little piece of tissue that connects the upper lip to the gum between the two front teeth. If it was tight and gets hit and stretched too much it will tear. It bleeds a lot at first, but will likely be fine after 20 minutes or so. Usually no stitches are necessary. In fact there may be no treatment needed at all. This is a very common injury in preschoolers.
More info on dental accidents here:
Dental Accidents (with links to related posts)