Buses don't goto the parks from DTD only to resorts. People have posted before about parking at DTD and getting a bus to a resort then another to a park, then same back at end of day. Its possible to do but would anyone seriously do that to save £8.95/day after spending £3,4,5k... on a trip. I love saving money but its either pay £8.95 or waste upto 2 hours a day messing with busses.
If they really want to save the parking fee either stay onsite or stay at an offsite resort with a shuttle bus(tips expected)