Has your youngest child in particular got a seat of their own?.
I would check the car seats at the check in desk, taking them on the aircraft may well become more trouble than it is worth (those aircraft aisles are very narrow). This is what they say about car seats on board:-
www.virgin-atlantic.com/gb/en/travel-information/flying-with-children/children.html
Also from VAs website:-
"If you are travelling with infants or children, you can bring a fully collapsible pushchair and car seat, in addition to their free check in luggage allowance.
Infants too young for their own seat on the aircraft (travelling on the lap of a parent or carer) can have one piece of hold baggage weighing up to 23kg, and one piece of hand baggage weighing up to 6kg. We’re afraid we can’t allow extra baggage allowance to be purchased for infants".
Again from VA re the buggy:-
"Pushchairs may be checked in or used up until the boarding gate. On landing, whenever possible we’ll have your pushchair ready and waiting at the aircraft doors. However, you may have to reclaim it at the baggage hall, due to local customs restrictions at some destinations".
At Orlando for instance these are usually left in the Oversized luggage part of baggage reclaim near the baggage carousel or on the carousel itself. If there are not many buggies you may actually be able to collect it upon disembarking the aircraft. You should be advised at the departure gate as to where to collect it; you can take the buggy up to the gate.
They will warm up the milk but do test it carefully before giving this to your children; it can be still hot.
Changing nappies onboard will be done in a confined space; there is usually only enough room to turn around. Carrying a small change mat is a must as there will not be one of those on board.
Do not rely on the airline to fully provide. Take a spare set of clothes incase of spillage.
You may have to walk your youngest child up and down the aisle of the plane. Wear flat shoes!.
Drinks are included but they tend to do only one drinks run. If you want more drinks you will need to go to their galley area.
There are USB ports usually in economy seats. The website "Seat Guru" should be able to give you more details about the planes Virgin use on their routes.
You will be given a white US customs form onboard or at check in to complete. This will need to be given to the Immigration Officer upon arrival into the US.
Leave any uneaten food, particularly fruit if you have this, behind on the aircraft.