15 month old fell off bed hit edge of piano leg. 1/2 inch cut to back curve of head. bled a lot but stopped. playful for hours. now asleep. worry?

Should be fine: If you are concerned-allow a wake up time in the next hour or 2 to see if your toddler arouses normally. This sounds like a low risk head injury that should improve as the next 24 to 48 hrs evolve. Tend to the cut/laceration to the scalp with soap and water and neosporin/bacitracin oinment. Serious head injuries would be hearalded by loss of consciousness, fussiness, unconsolable, nausea/vomiting.