Dr Rob Kidd, a retired biologist and lecturer in medical anatomy, sent me an email in response to the question. I enjoyed his scientific approach, so have printed it – with his permission – below:
“I speak here as a parent rather than as a biologist. I have two daughters, and in recent years, five grandkids. If I go through the slide library of the kids growing up, I can find hundreds of pictures of me sharing a bath with them, having nude water fights or whatever – and I firmly believe this is normal, harmless fun. I would suggest that it runs its course naturally and it is not a question of making a decision of when it is time to stop, but more that it stops naturally.