maroonmom: we drove from NC last year and did exactly what you are planning--left after school, drove to Kingsland and then in on June 4th--but our June 4th was a Friday.
I hate I-4, but when we arrived--about 11:30 am, there was no problem. I would hope that driving in on a Saturday would work.
If you get to Universal and find the traffic is just horrible, exit at Sand Lake, take a right off the interstate, then a left at the first stop light--this is a great road and I've never found it very busy--it turns into Palm Parkway--Palm Parkway comes into Apopka Vineland road just above Crossroads Shopping Center--near Downtown Disney. Good Luck!!