If you make the bookings at the same time this is definitely possible ( common scenario that I've done is when insufficient Avios seats so a mix of Avios + Cash with children split from adults on different bookings.) - you'd have to do this over the phone

If the booking is already made and designated as unaccompanied minors and you want to un designate it and link it to your booking then that might be more difficult .. not sure how easy that would be.