Last week, Ariel's Grotto didn't open til about 90 minutes after the park opened. The Fairy Godmother at CRT gave us a tip to get there right when it opened to avoid long lines. We did and were grateful because we passed by it several times throughout the day and saw very long lines.
The other nice feature to the Grotto is that since it's in a "cave", you can hear everything Ariel says to your child. I find that the princesses at CRT talk in such a low voice that I can never hear what they're saying.