I have witnessed a scenario like this on EY - granted SQ might take a different view - but in any case it unfolded like this: I'm seated in J, boarding ongoing. Man on his own boards, says he's seated in J, but would like to swap with his wife, who is seated in Y. No problem say cabin crew. Then the wife appears holding a baby! Cabin crew not happy - they tell her in no uncertain terms that either one of the adults can take the J seat, but the baby stays in Y. She agrees, takes the baby back and reappears alone (bet the husband hadn't bargained on giving up his J seat AND being left with the baby!). Shortly after takeoff, husband reappears with baby, leaves her with mum and disappears back to Y. CSM then appears and tells the mum that's it, everyone is sitting in the seat on their BP for the rest of the flight - go back to Y and please don't set foot in the J cabin again. Very awkward all round TBH.