I had the same experience with a dcg. She would smash her head to the point of nasty goose eggs. After trying several things, I found that not reacting to the head smashing and calmly securing her in a safe place (for me it was a high chair but a play pen would work) worked best. She eventually grew out of it once she became a bit more verbal ("help please" is the first phrase I teach my kids!). FYI, I had an agreement with the mom that I would text her if I felt she had hit her head moderately to extremely hard (for liability purposes).