Hi Carrie!
I'm sorry to hear Ariel's Grotto might be closed during your vacation. I personally haven't read anything official, so I'm hoping this might just be a calendar glitch on the site. Upon arrival, check the Times Guide at the front of the park to see if Ariel is greeting guests that day.
As an alternative option, I'd suggest booking a character dining experience at
Akershus Royal Banquet Hall in Epcot. Although characters are always subject to change, in our experience we've been able to meet and greet Ariel here on our last few visits. Booking Advanced Dining Reservations will be essential, so I'd suggest clicking the above link to determine availability. You can also call Disney Dining directly at 407-WDW-DINE, 407-939-3463.
Although not a meet and greet location, your princesses will also be able to see Ariel at the Celebrate a Dream Come True parade in Magic Kingdom and Voyage of the Little Mermaid or Fantasmic! at Disney's Hollywood Studios.
I hope your little Ariel fans have the most magical vacation, Carrie!