What about Hawaii? They only charge 60k for RT in F on UA metal. Transfer partner of everyone you have.

If you are flexible I bet you can find 4 seats together and potentially find lie-flats on the IAD / EWR direct flights (HNL only).

As an aside I would do a fair amount of research into Singapore KrisFlyer and sign-up for an account. All 3 of your CC currencies transfer to SQ (it's the only one common to all 3) so that alone makes it attractive when you are trying to book so many on one flight.

SQ almost always only opens long-haul award space to their own members so you need an account anyway. Do keep in mind that redeeming SQ for SQ flights comes with hefty fuel surcharges (though redeeming SQ for UA flights such as Hawaii results in nominal taxes and fees but no fuel surcharges).

Perhaps you could transfer the UR to Hyatt or Marriott (assuming you can cover the Hawaii flights using TYP/AMEX which shouldn't be a problem at 60k x 4) and could use that to book 5 nights at a nice Hyatt / Marriott resort in Hawaii making your out of pocket expense close to zero for hotels and flights which isn't bad for a family trip to Hawaii. Key is flexibility to find the 4 F seats on UA in the first place.