we've done Virgin and BA to HK (sister lives there). BA was better. Virgin would not let us have more than 2 bulkhead seats, so me, baby and DD sat in row 1, DH & DS sat behind us. We thought "fair enough, there must be another family needing a bassinet getting on". Oh no, a couple in their 50's who had booked bulkhead!
It would have been much nicer for us to have 4 seats in a row, as DD and DS wanted to play cards and things together, and I'd have had more privacy for breastfeeding. The cabin crew asked the couple if they would swap; they refused, which was stupid of them as DD is a real wriggler and spilt drinks from their tray-tables twice - while I was breastfeeding, I didn't goad her into it! So then I swapped seats with her and was right by the wife, who kept making pointed comments about "nursing mothers should bring bottled milk onto planes for their babies"! It was hell! (DH leaned over and swore at her, which shut her up...) We experienced something similar on the way back, and also on a trip to the US with Virgin - no family/group is allowed to pre-book more than 2 bulkhead seats (unless they have more than 2 babies with them.) Then other people, who don't necessarily need them pre-book the others, and at check in its too late to get them. The cabin crew were always helpful and apologetic over it. Don't know how they would cope with your situation, OP.
(This was all in premium economy, btw, but I can't imagine they have a different rule than that for economy.)
OP - with a 3-4-3 formation what you will probably end up with is you and 2 year old in bulkhead seats, with older DC just across the aisle slightly back from you, in the row just behind bulkhead, with an empty seat next to them. That seems like the obvious solution to me. There is no way your DC will get bulkhead seats that a parent and baby needs - even if you negotiate that now, then on the day at check-in they will be swapped around. You'll only manage it if there aren't other families needing bulkheads.