This is exactly what we did last year (although we didn't have kids!). We spend a few hours at TL in the morning went back to the hotel for a few hours and then went to MK for MNSSHP at 4pm! Since you can get into MK at 4pm we didn't want to spend a hopper day at another park and we wanted to take advantage of the 3 extra hours in the park (also to make sure we could do Splash Mountain a few times while the sun was still out).
Good plan!