Family of 5, multiple voucher – practicalities

[Sandydoc]

I’m looking to book next summer’s holiday. 2 adults and 3 kids under 12. We have 2 BAPP 241s (1 in each adults name) and 2 Barclays upgrade vouchers (in my name).
Looking to travel CW. May need to take separate flights to secure 5 seats eg 1 adult and 2 kids + 1 adult and 1 kid.
If we book 1 adult and 1 child on each 241 can I use the upgrade voucher for 1 child?
How do I book this in practice, does it need to be over the phone (I can’t book a lone ticket for a child online)?
Any other considerations/tips?
Aiming for vancouver (possibly with some use of Seattle depending on availability).
Thanks