Just a quick reminder, you have almost 3 months to help train your daughter in restaurant situations. I would be more concerned with ride queues myself with a 5 year old since you will be doing more of that than eating at sit down restaurants. I don't know your parenting style but I strongly recommend that you don't give your child an iphone or ipad in restaurant situations now or ever. You're better off discussing what you've done so far for the day, what you plan to do for the rest of the day, ask her about her school, teacher, friends, favorite shows or her dreams in life (which may change hourly!). Bring a coloring book or drawing pad and some crayons (in a plastic food storge or pencil box) and have her draw pictures of her experiences including your family. Have her practice writing her name. Look for hidden mickeys. Sing songs or tell each other stories.
Suggestions here are easy to come by, but it helps to know what she is interested in. Does she like gentle rides, thrill rides, parades, meeting characters, fireworks, princesses, super heroes or star wars? Is she a picky eater or does she enjoy trying new foods? Is she (and are you) morning people or night owls, or both? Can she handle long walks, and does she still take naps in the car on a busy shopping day for example?
Here are some very generic tips you probably already knew about:
Right now, advanced dining reservations (ADRs) are 60 days out. You can book them for your entire trip on the day which is 60 days before your trip is scheduled to begin.
Make sure to have reasonable expectations about what rides you will be able to fit in your trip, and prioritize those for a rope drop or genie+/ILL reservation if necessary. Nobody wants to pay extra on an expensive trip but if you're already dropping decent money here and don't get to do your must-do attractions it will be a much more negative experience.
If you drive yourself, parking at the magic kingdom is the worst of the 4 parks because you need to park, walk to the transporation center, get on the monorail and then walk some more just to get to the front gates. Taking breaks in the middle of the day from the other parks and going back to your hotel for a nap or pool break is good, but leaving and coming back to the MK adds a lot more time.
One general dining suggestion many here might agree with - the Be Our Guest restaurant in MK is quick service for lunch but table service for dinner. For lunch it's reasonably priced (as much as anything at Disney) and is a great way to beat the heat and have a sitting break. For dinner it may not have food she (or you) really like and it's considerably more expensive and for my family it just took too long.
Bringing a larger sling bag or sackpack (search on amazon if you don't know) with a change of clothes in a gallon ziplock or "space bag" might be a good idea along with the stuff Figgy1 mentioned. In May, a 5 year old girl's complete change of summer clothes doesn't take up much room but leaving resorts due to unexpected rains, or a food/bathroom accident is going to be quite inconvenient. Also purchase an external battery charger for your own phone and keep it in your bag, you will need it for ordering quick service food, looking for bathrooms, checking ride queues, and if you choose to purchase genie+ or individual lightning lanes for attractions. For genie+, people have been recommending getting them only for the days you're in the MK and DHS parks, but if you haven't been recently you should look up how they work because it's very different than the old fastpass system.
