We just did a 4 day WDW trip in July and went to St. Petersburg beach for 5 days after that.
While my kids had fun at the beach and there were no travel issues (there was construction but didn't slow us much on a late Friday afternoon), we felt like we lost something.
Personally, I'd scrap the side trip and concentrate on making the magic come alive in Disney. We didn't allow enough time and are going back this summer!
But no matter what you choose, have a magical trip!