I've been in this similar situation many times albeit several years ago and had no trouble - though it was admittedly never on HP. My understanding is that it supposedly depends on the childs weight and I've heard that others have at times been directed to hold the child. I'm afraid it may well be determined by the head FA responsible for your particular flight! Incidentally, a two year old with no shoulder strap, but just a seatbelt is not in the best shape either. Hope you make out well in any case!