My kids are now 14 and 12 and they don't know what it is to fly long haul Y.....yet. I would recommend upgrading every time with young kids as whilst they do appreciate it you will. Freedom of movement and better service generally will make your journey less stressful. Infants on board are always a challenge for any patent so having space and help can be a real boon.
Kids meals on board are poor and BA don't do kids in F well so you may need to ask the crew for some help with things like China cup and glassware. Cups and saucers are not the usual tableware fora three year old and it may make your life easier if they don't have them.
Headsets for the IFE are another weakness as they are too big for most kids heads.
Finally, it rare nowadays but there is more chance of a spare seat in F which can be helpful if using the cot position and thus losing the IFE.
The best bit is using the door to the Concorde room just watch the reactions.