Pools are not just for kids ...
The latest episode featured a decent-size above ground pool as a vehicle to show the kids becoming adults. They lost all interest in using the pool. But since when do only kids enjoy the pool? Mike should have led by example.
That said, they never should have gotten the pool in the first place due to their money woes. They are perpetually broke (Frankie's card got declined that same episode) and pools cost money.