DS (2.9yo) and DNiece (3.6yo) started arguing over a toy. DS smacked DN, DN bit DS (hard - bruise came up immediately). Result was two screaming children. :(
DS "started" it, but DN's response was a bit disproportionate (the bruise makes me feel I'm not just being all PFB about it - if she'd just smacked him back I may have even said "serves you right!").
How would you deal with this situation? They had been playing beautifully immediately prior to this!