Heathrow does not allow gate passes , they will be escorted through security by the UM co-ordinator or a contractor at LHR on the day.

On arrival in the US you wont be able to go to the gate as you will not be able to enter customs and immigration when the child clears

It used to be that you were given a gate pass for one of the domestic gates and the children were escorted there after clearing customs.**

On the return trip you will be able to go to the gate with them until they board the aircraft.

** policy may well have changed . I believe now that the children are simply taken through customs and you meet them where everyone else comes out and does the reunion thing.