At WDW, time is money. Some things save you time, some things save you money. Disney knows this inside out!
Usually, if you pay more, you get more. Deluxe hotels are usually closer to the parks, which saves time. You pay extra, but save time.
Minnie vans/taxi cost more, but when you are crunched for time- the time savings can be very helpful!
Meanwhile, many money saving ideas costs time. Carrying lots of stuff on your back all day - food, extra clothes, jackets, toys...can save $, but all the extra weight all day tires you out, and therefore costs time.
DO carry water! Staying hydrated is super important, especially in summer! Refilling your water bottle is good!
DO carry jacket or rain ponchos if cold or rain expected.
DO ALWAYS carry emergency MEDS! ID, and insurance cards!
Otherwise, try to carry a minimum in the parks. Clean out your purse: you won't need your library card at WDW.
Whenever possible- minimize! With a baby, DO carry diapers/wipes, but only as many as you need each day. A small tube of chapstick-style sunscreen is enough. You might want a few over-the-counter Tylenol/Advil, but you don't need a giant bottle.
Any item that does double duty is good. Cell phones serve many functions in one: camera, phone, electronic documents, etc. Pictures are an easy way to keep track of many things- like where you parked your car at the airport/parks, hotel room number (If phone is locked), etc. Also, what your kids are wearing each morning!
Ziplock bags are super useful!
A tiny sharpie pen has many uses! (they are about half the size of regular Sharpie!) Sharpies can be used to label almost any plastic item. So: names on refillable mugs, marking the remaining balance of giftcards, etc.
A small tube- 1 ounce- of hand sanitizer is great! It does triple duty: it erases Sharpie on plastic (not fabric), it sanitizes hands, and disinfects cuts. Washing hands is always better, but washing isn't always feasible. All the WDW parks have first aid stations, but that might be a ways from where you are when you scrape your knee. The alcohol in hand sanitizer also calms many rashes and bug bites. (though few bug bites at WDW!)
Oh, September in WDW is Love bug season. They are harmless, just plentiful little black/red beetles. They are called love bugs because they show up to mate.