Not much chance for a law sue. My son had many similar accidents in day care and school. His forehead once had 10 stitches (head hit the door after a girl pushed him in the school).
If you worry about the scar on his face, here is what you can do
1) Aviod direct sun (put at 70 sunscreen on his scar every day)
2) Apply scar cream (buy from CVS, Walgreen)
3) Apply Silicone Scar Sheet to the injured place (at least 7 hours a day), it will smooth the scar slowly.
Hope these help.