If you are staying offsite, you will want to avoid AK on Saturday as it has 'Extra Magic Hour' early entry for WDW resort guests that day and will be busier. Also, sadly, it no longer seems to open earlier than the other parks - at one time a great reason to visit it on the first day for us jet-lagged Brits. You will all be awake at the crack of dawn and raring to go, so make the most of your first day and just call it a day when you've had enough.
Although MK is normally best avoided on a Saturday, if you get there before park opening (which will be easy on your first day), you will be able to get through loads before it starts to get busy. With your girls being the ages they are, head straight to Fantasyland and do everything there first - it will be a doddle (and hell later in the day).
Remember that you need to head to MK a little earlier than the other parks (because you park at the Transportation and Ticket Center and get the monorail or ferry from there). If park opening is at 9am, aim to arrive at the TTC by 8am to be first through the turnstiles. This really won't be a problem on your first day so, as I've already said, make the most of it.