Pack feminine supplies, cold medicine, headache relief, sunscreen, sunburn lotion, benadryl, extra socks, and an extra pair of shoes. Also anything baby related - have diapers sent via a grocery delivery service or just ship them down. Remember chargers - or batteries if you still have camera or something that needs them. All that sort of stuff is pricey at Disney.
Bring a water bottle, and if you don't like Disney's tap water (its a little sulphurous) a water additive like Crystal Light or Mio. Bring ice water into the park with you in the morning and refill with a cup of ice water as needed.
By an inexpensive raincoat or camping poncho and if traveling in cooler seasons, remember you'll want long pants and jackets and sweaters. Dollar store ponchos aren't worth even the dollar, and Disney ponchos are expensive.
Bring snacks into the park - granola bars, nuts - if its hot, don't bring anything that will melt in the heat. Also consider "breakfast" in the room - even if its cold pop tarts. Extra meals at Disney are expensive.
Definitely bring a credit card - and consider ordering from
Amazon if you forget something you need. By tomorrow, Amazon will ship you new underwear - you may have to rinse out the pair you are wearing. My husband has traveled all over the world on business and says "if I have a credit card, my phone, and my passport....it may be expensive, but they can loose my luggage and I'll be fine."
On the loose the luggage front, if flying and using ME and checking bags, pack a carry on with a change of clothes, spare underwear, sunscreen (less than 3oz!) and a swimsuit if you want to swim. You won't get your luggage until later, and if it does get lost, you can get yourself through a day.