Hi Jennifer! Thanks, we love helping out here at the Moms Panel so it's nice to know it's a big ol' love fest going on!
First of all, happy early birthday to your daughter - I can't think of a better place to turn 4 than at the most magical place on earth!
1900 Park Fare is one of my favorite character dining locations too. While it is not *guaranteed*, there's been known to be some birthday singing going on at 1900 Park Fare. I would just mention it again at check-in
and to your server when you are seated. It can't hurt to tell as many people as you can or even ask specifically. Again, while they can't guarantee it, it will probably be hard to resist a cute 4 year-old's birthday request. ;) And if they happen not to be able to sing, your family can always lead the way by singing to her yourselves.
Have a wonderful time, and I hope your daughter's birthday wishes come true! ~Suzannah