Hi Viviane! What a special date night you're planning! How exciting that you're dining at Victoria & Albert's! Did you know this restaurant has its own website at
http://www.victoria-alberts.com? You can take a peek at sample menus there!
If you use Disney transportation to reach Victoria & Albert's, the trip will take you about an hour. You'll need to take a bus from the All-Star Movies to the Magic Kingdom, and then take the boat or monorail over to the Grand Floridian. Since you'll have to leave your resort by 4:30 at the latest, you'd probably want to make sure your babysitter arrives by 4 p.m. so you can take a few minutes to talk with her and help your children get accustomed to her, and then have time to wait for the bus to arrive.
If you take a taxi to the Grand Floridian, you could get there in 15 or 20 minutes, which means the sitter should arrive by 5 p.m.. I generally opt for Disney transportation over cabs because I can be frugal; however, in this situation, I would take a taxi. You'll be dressed in your fine clothes for your special meal at Victoria & Albert's and you won't want to be running in your dress and heels to catch a bus or monorail. Also, since the taxi ride will be so much faster than Disney transportation, you won't need your sitter for as long. Since you'll have to pay the sitter for less time, the cab ride pays for itself!
Hope this helps! Have a magical time!