Because people take criticism of their parenting (in this case lack of it) very personally, I would probably just have moved to a different area of the lounge.

If I had really felt moved to comment I would have done so via a BA staff member who could politely ask the parents to take their kids into the kids area or ask that they do not disturb other lounge users.

I have two kids (2 and 4) myself and it can be hard keeping them quiet and in one place during a long layover or a delay. That said, I always take them out onto the CCR or GF terrace when in T5 and keep them under control.

If they are no longer happily distracted by books or an iPad, and need to run around, I take them round the corner of gate A8 where there is a kids play area.