Possible clue to OP's route is their username, as dnw has figured...

Assuming OP has booked 2xJ/U for 1 parent and child, and will book the W/P for other parent and child, I wonder if OP's chance of getting a POUG is affected by having a child in the booking.

Can't upgrade an avios/P booking to U with cash but can upgrade a WET cash booking into U with avios, and I've observed as long as there is "I" inventory, doing an avios upgrade into U does not remove U space, as long as U is >0, so it's possible to have more than 2 Us, even ex-MAD.