Talk to the mom. Only you can decide if it is a situation of abuse or not. If you suspect abuse after talking with mom you are required by law to report. Either way, I would photgraph and document the situation, discussion with mom and any follow up.
FWIW~ I accidentally elbowed my DS when he was 2.5 yrs old. He was constantly following me around like super close. He got a HUGE black eye out of it. Whenever someone asked what happened, he would point at me and say "Mommy did it." It was true though, I did.
