If the 2 kids were younger, it might be worthwhile to go for the character meal at Ariel's. We did Ariel's for lunch a couple of years ago and honestly, the character interaction was very minimal. We had more interaction with the princesses at Princess Fantasy Faire in Fantasyland...and that doesn't cost you extra. At the Ariel's Grotto character meal, you meet 4 princesses, but it's really barely enough time for them to say hi and take a quick picture before they go on to the next table.
Compare that to my kids' character interaction at Princess Fantasy Faire in June and October - the girls had a full on CONVERSATION with Cinderella, Rapunzel, Snow White, Sleeping Beauty, and Merida (not all 5 at once - I think there's 3 indoors and 1 outside, or maybe it's 2 inside and 1 outside, I can't remember).
Then there's the Anna & Elsa meet & greet at DCA. That was awesome, too. Anna & Elsa are not present at any character meals.
If you'd like to spring for a character meal, do Breakfast with Minnie & Friends at the Plaza Inn. It's a really great value and you often meet at least 7 characters during that breakfast. Sometimes people can meet up to 10. No princesses, though. But the character interaction is really great...if you engage the characters and play silly games with them like patty cake or talk to them about themselves, they respond in really cool ways. Tigger is a hoot. Rafiki was awesome, too, as was Captain Hook, the Fairy Godmother, and a bunch of others I'm forgetting right now. We got lots of kisses from all of them.