WE did this for my dd's birthdays in May. I don't remember the number to call, but it is the same one for dining reservations, it's on the
Disneyland website.
I totally recommend doing early make-overs and then lunch, you need at least an hour and a half between the bbb reservation, and the Ariels reservation.
The makeover takes between 30 and 45 minutes, if you are getting a dress and accessories, or pictures it takes longer.
We took the first reservations in the morning at bbb, and then the girls got to be princess all day.
They loved being dressed up at Ariels, and they got lots of attention between bbb and Ariels, as we had time for a few attractions between.
Also, we checked in at Ariel's at the time they said, and still waited more than 1/2 hour, but it was Mother's Day, you might want to consider that when making the reservation, you may have to wait a little, I know little ones get hungry and need to keep a schedule, even at Disneyland.
I found the # it is 714) 781-3463. The cm I talked to handled all of my reservations at one time, as well as got me to the right place for our in room gift baskets.
I also recommend getting a reservation inside at Ariels for lunch, as they do a ceremony where they announce the princess's and a birthday girl gets to help, be sure to tell the hostess you are celebrating a birthday, they do choose at random though, and you can't see the ceremony from outside.
Dinner at ARiels is for WOC, no princess, so make sure to go for breakfast or lunch.
Hope that helps.