Almost any of the resort Hotel restaurants would be nice.
Some of my favorites:
Narcoosee's
Jiko's
Citricos
California Grill
Artist Point
Shulas
Palio
Yachtsman Steakhouse
Over at the Boardwalk is the Flying Fish. At Downtown Disney, Portobello yacht Club can't be beat.
Mothers day is a huge dinner day. Be sure to make PS just as soon as you decide.