I don't know about Virgin but when I had UM (unaccompanied minors) I didn't "make" them do anything. I just checked on them more often. We usually seated UM's together anyway, even if they weren't traveling together. Do they have their seats already?

Can't talk about U.K. Customs but the ground agent will keep the UM's with them.

Both girls need their American passports for entering the U.S. The girl with Finnish nationality can't use it to go home but she can use it to enter the U.K. They might do better just staying in the same line together. I've even asked foreign passport controls which passport they want.

The time difference is only 6 hours and it'll be worse returning to the U.S. They need to try to get their bodies in the new time zone as fast as possible; go to bed at bedtime, don't sit up using the computer, iPhone or watching T.V., get up at some sort of morning time, get out of the house or hotel room and be outside with other people... I'm a big melatonin fan and if they want to use it, they have to buy it in the U.S. and take it with them.