As a mom of 3 (soon to be 4), I absolutely know that bumps and bruises are going to happen. And I hope that my clients know that, too. Obviously, the more "seasoned" parents are a little more understanding of it, while first time parents of the 10-12 month olds are still getting used to it.
My general rule is; any injury that is going to leave a mark (bruise, scrape, etc) gets written up via an incident report. I know it seems like a lot of paperwork, but I would rather have it in a file than have a parent screaming at me later.
I have not had to call a parent to come pick up their kid in response to a dayhome accident, thankfully. I think I would just use my judgement in that case. If the child was utterly inconsolable even after 15-20 min of comforting, or if I thought that medical attention was necessary (but not an emergency).