We took my friend’s 5 year old girl for their first trip last summer, so I have some experience with this!
We booked the BBB and she loved it! We bought a princess dress before we went (you can get them at Walmart or Target and they’re pretty nice ones) so that cut down on the cost a lot. We just bought the package with hair, makeup, and nails. Afterwards, I highly recommend going to the photo shoot. If you have a photopass, you can put all the pics on that instead of buying them there.
We went to MK two days early in the morning. Once was to get Fantasyland accomplished before the lines got bad and the other time was to meet the fairies before the line got bad. I’m not sure where the fairies will be when you go, but I would highly recommend doing them first thing in the morning.
We booked a princess breakfast at Akershus. It’s cheaper than CRT and Emma loved it (and she never knew the difference because we didn’t tell her she could have eaten in the castle). The character interaction was great and I love the food there for breakfast.
Each day we had a surprise for Emma when she woke up in the morning. It wasn’t always something huge, but just a little something to get her excited for the day. We booked the princess breakfast the day before the BBB, so on the morning of the breakfast we surprised her with her dress so she could wear it to breakfast. I would definitely recommend getting a princess dress for them to wear to the breakfast. Almost every girl was in one and I know it makes them feel special. We didn’t tell her about the BBB, so that was a surprise for the next day. You get the idea. Just keep a few little secrets to surprise them with.
That’s all I can think of for now. Otherwise, just remember what it’s like to travel with little ones. They do have meltdowns and do need breaks in the middle of the day. I was prepared for this, but it’s still a totally different way to travel than when it’s all adults.