Keep in mind that Citrico's is one of the most adult oriented restaurants at WDW, probably the most other than V&A's.
The service is leisurely, plan on minimum two hours.
As the other poster suggested, if your child won't be happy sitting in the highchair for two plus hours, you might want to get a sitter or wait. Frankly I have several young neices and nephews with varying levels of manners and tolerence for drawn out events, and I wouldn't consider taking the extremely well mannered six year niece old to Citrico's--not that I think she wouldn't behave, but because I think she'd be happier elsewhere--it's just too long of a meal for most children.
Jel gives some great advice about scrapping plans if the kids are cranky, and bringing lots of diversions. I'd also add that you should let the server know immediately when you are seated that you'd like the service to be a little faster than normal, and be ready to leave if the child gets at all disruptive.
Scott is right on about the dress code, while you don't need to wear a suit and tie, "business casual" is requested, and seems to be the norm for dining there.
My personal thoughts on it--if I were in your shoes, I'd get a sitter and take DH out for a nice leisurely dinner with just the two of you, adn follow it with a nightcap at Mizners
Anne