BSD -Number one – I can’t believe any Rov told you- you can sue. You can take them to a Din Torah. The only way you can win in a Din Torah is if an adult hurt the kid, not another kid or your kid hurt herself.
Second of all -Did a plastic surgeon put in the stitches? Lots of times these wounds won’t scar -they are very good at closing wounds.
Third – If after they remove the stitches – if you use an OTC scar treatment, it might not scar. The best is one that is like a hard plastic covering.