I don't think so. We make a lot of trips to MK (with a 3 year old) and usually have a car; but we ride the bus to MK (and drive to the other parks, etc.)
We find that after the bus leaves the OKW grounds, there are no other stops and you head straight to MK (I think some of the buses at other resorts are shared, and may make some stops before going to MK). Also, you get dropped off at the "bus terminal" fairly close to the gates.
So , I don't think it would be any longer of a bus ride than a car ride; and you end up closer to the gates.
From the "unofficial guide book" I have, it also indicates that riding the bus to MK from OKW is usually faster than driving.
Trudy