Having a little red head who LOVEs Ariel, we've seen her many times in her grotto.
In my experience, it has sometimes opened the same time as the rest of the park but sometimes, for some reason, there is a delay by 1/2 hour to an hour.
The line gets long fast so IMHO, if you really want to see her, do it first thing in the AM if you can. The line is also slow moving and makes frequent stops for breaks. Luckily there's stuff for the little ones to entertain them there. Water sprays out at certain times of the year.
They also added more umbrellas so not as much of the line is in the sun as it used to be.