They only release two reward seats in Premium Economy I think.
There are plenty of days with 4 club available, but you don't have enough Avios for that (and the prices are insane £18k to book W and upgrade to J for 4 in Jul 25)
So you can book 2 upgraded Y seats in W and book 2 Y only for cash on another ticket. As you are gold you can get them linked to give you seat selection on all of them.The kids can sit in the back