The $90 is the same as the two taxi rides to/from Manhattan. Give yourself an hour on each end, another 30 minutes for security, and 45 minutes to be at the gate before departure, you still have almost four hours in Manhattan. I've spent time in way too many SkyClubs and seven hours is way too long.
I know your child will be perfectly behaved in the SKyClub, but something tells me your spouse will enjoy shopping more.
Get out and see the city.