Where are you staying and how will you be travelling around? One thing you might not be aware of as this is your first trip, is that (assuming you are staying offsite and arriving by car) it takes a little longer to get to MK than to the other parks.
You will arrive in the parking lot of the Transportation and Ticket Center. From here you will either need to walk, or catch a trolley (depending on how far out you have to park) to the ticket booths. If you haven't already exchanged your vouchers at Downtown Disney, I suggest you do it here. From here you will need to get the monorail or ferry across to MK. If it's not too busy, I suggest going for the monorail. If a ferry is in, or about to dock, you may be quicker taking that. There won't be a huge deal in it either way, so don't fret too much about that one.
Once you alight you're there! You will have to pass through the security checkpoint (you might save a bit of time by making sure you don't have bags - you can then walk straight through without waiting in line) and the turnstiles and then it's just a short stroll up Main Street to the Castle.
The official opening time when you are there is 9am. As it's a busy time, they may well open earlier than the official time (maybe as early as 8am). Even if they don't, the TTC will be open long before that. You should aim to arrive at the Castle 15 minutes before your PS time (as they ask you to do for all PSs). If the park doesn't open until 9am, just tell them at the turnstile that you have a PS at 8.50am and they will let you in. As Allie says, you will all be awake at the crack of dawn on that first day, so I would aim to arrive at the TTC at around 7.30am.