I had an identical situation like this happen. My ex and I had shared custody of our children at the time. He took the kids out one night when they were with him and my youngest got hurt at the restaurant. The restaurant called an ambulance since it was a head injury (required stitches), and my injured child, my ex AND my other children were all taken by ambulance to the hospital.
I wasn't notified until the next morning, and that was when my oldest called to tell me what happened. She said that Dad wouldn't let her call me when they got home because it was late. Granted, cell phones weren't big back then, but I did have a pager and he didn't bother paging me either. (keep in mind, he called his parents to come get him and take him back to his vehicle...)
I was extremely pizzed, because I felt he could have at least called me to be there for my child, be there for my other children that were with him; not to mention the fact that I also carried the insurance on the kids.
I didn't cause a scene when I found out, but I did make it clear to him that if this happens again in the future, I am to be notified right after 911 had been called, and that I would show him the same respect.
It's all about respect. The other parent should be informed of the situation and they should have the opportunity to choose which course of action they want to take.