Hi Amanda,
I cannot think of a better start to a day at the Magic Kingdom for your two little girls than visiting Cinderella's Royal Table for breakfast and meeting the Disney Princesses!
If your reservation is for 8.30am, I would be aiming to be at the Magic Kingdom parking lot on that particular day for around 7.30am.
As the Magic Kingdom parking lot opens 1 hour prior to park opening, you will firstly need to check the Magic Kingdom park hours for the day of your visit by looking at the Park Calendar
HERE
If there are extra magic hours on this particular day, the parking lot will open 1 hour prior to the start of the extra magic hours.
If the parking lot will be open at 7.30am, simply make your way there and then catch the monorail or ferry boat across to the Magic Kingdom before heading to Cinderella's Castle to enjoy your breakfast.
If the parking lot will not be open at 7.30am, I would suggest that you drive directly to Disney's Contemporary Resort and explain that you have an early morning dining reservation in the Magic Kingdom and that the parking lot does not open in sufficient time for you to get there. You should be allowed to park your Car at the resort for up to 3 hours which will give you ample time to get to your reservation but will mean that once breakfast is over, you will have to move your Car over to the Magic Kingdom parking lot for the remainder of the day.
Once at Disney's Contemporary Resort you can either take a 5 minute walk over to the Magic Kingdom or catch a monorail from inside the resort itself.
I hope that this has provided you with 2 alternative plans and wish you all a magical holiday!