If you are really interested in seeing Princesses or Cinderella, you could book a lunch/dinner at 1900 Park Fair (Cinderella, Prince Charming, Evil Stepsisters and Stepmother) or at Akershus in Noway in Epcot (rotating princesses). Your 18 mo DD might enjoy either of those. Since you'll be at MK, 1900 PF is at the Grand Floridian so it's a short monorail ride away.
Other options would be Crystal Palace, which is in the MK and has Pooh characters or Chef Mickeys at the Contemporary...your daughter would enjoy either of those. If you're opting for a more adult meal experience in the MK area, there are several 2 credit Signature places Cali Grill (you can see the fireworks), Narcoossees at the GF (also good view of fireworks), and Citricos at GF. Nice 1 credit places include The Wave at CR, Grand Floridian Cafe, and Kona Cafe at Poly. I've heard Ohanas is a good meal, but can be a bit rowdy so if you're looking for a relaxed meal that may not be it.
You have tons of options if you choose to change! IMO Cinderellas Castle is good to do once but it's not really worth the 2 credits. They'd be better used elsewhere, either signature dining or adding a 1 credit and adding a breakfast.