Two Seats Question

Hello,
Long time lurker, first time poster here!
Quick question for the Virgin Brainiacs please ...
Booking flights to Orlando for Christmas and my husband and I are going to be in different countries before we fly. Depending on work schedules, one of us will have our eight year old daughter with us but we wont know which until about a week before Christmas.
My question would be ... if we booked one adult and one child ticket from London to Orlando and one adult and one child ticket from Cape Town to London, would we have to cancel the child ticket if she doesn't fly or can we keep it as a spare seat?
Many thanks for your help.
Long time lurker, first time poster here!
Quick question for the Virgin Brainiacs please ...
Booking flights to Orlando for Christmas and my husband and I are going to be in different countries before we fly. Depending on work schedules, one of us will have our eight year old daughter with us but we wont know which until about a week before Christmas.
My question would be ... if we booked one adult and one child ticket from London to Orlando and one adult and one child ticket from Cape Town to London, would we have to cancel the child ticket if she doesn't fly or can we keep it as a spare seat?
Many thanks for your help.