2-4-1 FTV voucher with three ticketed travellers – who “must” travel?

I have a 2-4-1 voucher issued in my name wrapped up in a FTV. The original booking was for myself, my wife, and my son (older than 2, i.e. paid full avios, full taxes).

We’ve got another little’n on the way and some other 2-4-1s, and I’m trying to work out which combinations of 2-4-1s is the best for future bookings for a (soon-to-be) family of 4.

My question is: who HAS to travel on my 3-person FTV with a 2-4-1 wrapped up in it;
– Are we locked in to all three of us travelling on that voucher (and need to add the new little’n later [10% avios/taxes])?
– Can I determine who the second traveller on that 2-4-1 is myself, and “release” the third person from the booking, while still using the full value and 2-4-1 for just myself and EITHER my son or my wife?

TIA