Spur of the moment 1 day at Magic Kingdom during Spring Break. What do I need to know?

My family (wife, 3 year old, 1 year old) is currently down in Palm City visiting family. I’m thinking of surprising them with a quick trip to Magic Kingdom on Saturday. Anything I should plan ahead for? Would basically passes work or do I need some time of fast pass? I’m Disney novice, so go easy on me. For what it’s worth, I imagine my wife and I will trade off doing ride with our son while the other stays with the 1 year old.