That has nothing to do with Kids sail free.
Rooms that hold 4 have higher prices for the first 2 guests than rooms that only hold 2 guests so even with kids sail free the rooms are still more than a room for 2 people.
If you want to see the difference KSF makes then compare the room cost wit 4 adults vs 2 adults and 2 kids not to a room with one or 2 in it.