It's likely that high because it's a one-way redemption and the corresponding one-way UC cash fare seems to be booking under C which is not one of the discounted UC fares.

If you book it as a return the cost drops from $7,700 to $3,100 as a cash fare. I wonder what would happen if one of the adults booked a return award using miles, added the lap infant (10% would now only be $310) then cancelled the return leg at a later date.