Communication is the key here. Talk to the mother of the child and explain how you feel. By ignoring her, or giving the cold shoulder shows an immature attitude. Has she asked her child to apologise (if she's old enough)? Tell her you realise kids will be kids, fights and accidents will happen and you don't bear any resentment. Ask her how she feels about things and afterwards forget it happened. It's not worth losing friendships over fighting children.