You might also want to check with your hotel regarding what time their buses to the parks start. The MK bus will drop you off at the TTC, so you'd need to get on the monorail anyway - but your ride would be faster because it's only two stop to the GF instead of four and almost a full circle.
I don't know absolutely that the Hotel Plaza Boulevard buses run that early (although if it's EMH at MK, since the Hilton guests get to participate it'd make sense that there'd be transportation early enough), but it's another option worth investigating - and you wouldn't need the car seats.