Hello Junne,
Celebrating a birthday at Walt Disney World is so much fun, and you are going to be able to celebrate two birthdays, so it should be double the fun!
First, I suggest that you pick up a "Happy Birthday" button for each of the birthday children. The rest of your traveling party can also get "I'm Celebrating" buttons to show that you are all a part of the celebration! These buttons are available free of charge at many Walt Disney World retail locations, as well as at Guest Relations. When wearing a happy birthday buttons, it is really fun to hear other guests and cast members alike wishing the birthday kids a happy day. Sometimes, the happy birthday buttons can help to encourage a little extra pixie dust, like a complimentary birthday cupcake. This type of pixie dust is not guaranteed, so please do not expect it, and it will be all the more magical if it does occur. I am not sure if your dining reservations are at Walt Disney World resort, but if any of them are, do note on the reservation that you are celebrating birthdays. Again, this does not guarantee any magic, but it would be fun if some does occur. You mentioned that you have dinners planned, but there are many great options for celebratory breakfast or lunch at Walt Disney World as well. Celebrating with Lilo and Stitch at
'Ohana at Disney's Polynesian Village Resort for breakfast would be fun, as the restaurant is family-style.
I often suggest letting the birthday children be totally in charge of the activities on their birthday. Before you head to Walt Disney World, have the birthday children work together to select the theme park that they want to go to on their birthday and what attractions they would like to do. Also, it might be fun to have your birthday children wear matching shirts on their birthday.
I hope that these thoughts are helpful for you, Junne!