If you are starting cruise in miami or tampa then there are lots of other great things to do there with kids. 3 hrs in the car possibly longer is no sort of fun for a 3 or 7 year old. hope they enjoy the cruise.
Would not personally do a three hour drive (I presume this is each way) in order to arrive at WDW. It will become a very long day for you all.
Day passes do exist; they work out a lot more expensive than if you were to buy a pass with more days on it. IIRC a day pass is around $89 each (it costs less for children). Car parking is $14 per vehicle at WDW.
We will be in Florida for a few days waitin to start a cruise. Rather than just shopping I was thinking of driving to magic kingdom AO out 3 and 7 year old daughters could experience Disney for the day. It's a 3 hot drive from where will be staying. I have been several times but not since I've had children (so its been ages). How muh is a day pass, do they even exist? How much is parking. We will be goig in February. Any tips (especially money saving) appreciated. Thanks.