How exciting! Do it. Have just returned with our just-turned 4 princess and 2 year old brother. It is exhausting but magical, she will love it. 6 months is easy, as the queues will not drive him mad, he will sleep. The 2 year old and queues were tough.
My tips: get fast pass tickets or get some costumes. The queues are painful for the kids. If you have the costume for the ride character you get 'fast pass' for that ride. (Buzz light year etc).
The princess pavilion queue is torture. If you do it, load up with snacks, books, colouring etc. It will be 1.5 hours of the most boring of alleged of them slow moving queue inside with nothing to look at.
Stay in one of the hotels and get up early. Hit breakfast first thing, or rush queuing an hour for buffet breakfast. You get 'magic hour' to visit rides 9am before day trippers come in at 10am.
Download an app that gives you real time queuing data, it will save you HOURS. Make sure you have data on your phone for your trip, makes life less stressful.
Don't miss the parade 530 ish daily. Pitch up half an hour early (with snacks) for a good position by the magic castle.
Be prepared for expensive food. We thought it was nice - cagey Mickey, rainforest cafe, lighter lunches, but cost us £200 in meals for 3 days family of 4!! Just brace yourself and go for it.
You will have a ball, perfect age.