To echo what others said...
If your list of "must haves" is long, I fear you do not understand the meaning of "
must have."
Toiletries and toothbrushes. Clothes (1 outfit per day, + 1, swimsuit, undies, socks, PJs) and shoes (1 pair of sneakers, one pair of sandals). Prescription medicines. ID and currency. For a baby, a stroller or carrier and diapers/formula/bottles. These are relative must-haves. Everything else is a frippery (or can be borrowed or purchased on-site).
My family of four travels with a single 21" carryon suitcase per person (no need for checked luggage at all), which holds all of their clothes and sundries (PLUS breakfast foods for the hotel room) for a 5-6 day trip. We also bring a backpack or small tote per person for that person's entertainment items, sunglasses, and other things for the journey. If we're staying for longer, I add a laundry bag and detergent pods, and do laundry on our mid-week pool day. (WDW puts their laundry rooms right by the hotel pools, and you can effortlessly monitor the progress of your washer/dryer via a website, so you know the minute it's done. Genius!) I, too, used to pack everything but the kitchen sink, until I had a job where I needed to travel frequently, and quickly learned that there is more freedom and joy in what you leave behind that the amount of stuff you take, and that mishaps with checked luggage can practically ruin a trip, so it's a risk best avoided.
My best advice: make a packing list, trim it to the bare necessities, and stick to it when you pack! If, God forbid, you find you left something at home that you really need, you are in a major tourist area in a developed country surrounded by Walmarts - it's all there at your fingertips.
...another tip: others suggested ziploc bags for kids' clothes, which is a great suggestion. I'd add, make "outfit burritos" for the kids. (Fold t-shirts in thirds and shorts in half -- place short waistband on top of shirt neck, put socks and underwear on top, and carefully roll it all together with the shirt on the outside, smoothing out any wrinkles as you go. Then place the "burritos" side-by-side in a packing cube or gallon-sized Ziploc bags. To unpack - simply lay them in a drawer, and the kids can get out a complete outfit for themselves in the morning with no guesswork and no temptation on your part to throw in extra, unnecessary clothes when packing.)