So it sounds like you already have two seats together for your wife and yourself. So worst case scenario (if they can't arrange three seats together) is that one of you will swap with your child. Child will then be with an adult (and the other adult will have a more relaxing journey in another seat
)
As its WTP we both took an aisle seat (so one window is free and one middle is free) so she could have either one, but now TS has blocked those, it will be a free for all at check in