I'm not 100% sure, but I beleive that as soon as one person in your party has seating availbility booking, all members should be able to get a seat. This will mean that your husbands booking will be good at t-72 for all of them to get a seat. You will have to wait until T-24 with your 12 year old.
You can keep and eye on the seat availability via the MMB pay for seat selection. You can view seats without having to pay for them.

I may be wrong on the first sentence? The reason I think that it may be true is because I have an infant, and when I book flights I am always able to get seats straight away, for not only my infant and me, but also for my wife who is also on my booking. I would assume (maybe wrongly), the same would apply for your husband's whole party, as it would not be very useful to have some children in the party sat away from others in your party.

One more option is to wait for t-72, get your husbands party booked into seats and then pay for your party seating next to them. That way you only pay for you and 1 child.

Good luck

Matt