As long as the sub-15s are in economy, one of the adults needs to remain behind also. Minors can not be in a different cabin from adults (not always enforced but for younger ones, it is more likely to be enforced.
Are these mainline or UX flights?
But first, 4 on PNR will not work
If a reservation includes three or more travelers, and only one is not a Premier member, the Complimentary Premier Upgrades will be processed according to the lowest Premier status level in the reservation.
If two or more travelers on the reservation are not Premier members, it will not be processed automatically. In these cases, Premier members should call the
Premier Priority Desk to request a separate reservation for themselves and the eligible companion.
2-2 with an elite on each will be needed but the Plat's chances will be lower