It's funny to see how people respond. YES, your hands will be full...with a 4, a 3 and infant twins, your hands will always be full LOL. But 2 adults can handle 4 children safely and have fun at the same time. Your family is what it is...will you never go on vacation? (It will be even crazier when the twins are crawling or walking *lol*)
Here's my suggestions -
1. Watch the heat with the babes, but hopefully Sept won't be too bad.
2. You WILL NEED to double strollers. Since you will have a car/mini-van, I suggest bringing your own. It will be useful travelling from the parking lots to the parks. One parent pushes one stroller and another pushes the other - no problem!
3. Bring 2 baby bjorns!
4. Have a very relaxed *schedule* and make sure everyone gets enough rest (best you can with infants
anyhow). I mean sleep in in the am, and either take afternoon breaks or call it quits early in the evening and just come back and swim.
5. Plan sit down meals or eat at your suite. Counter meals can be challenging with a bunch of little ones in tow. Buffets are good because you can get food right away for the little ones, but it's still a sit down atmosphere.
The part I would think is most difficult would be using the bus system (getting 4 kids on and off, not enough seat sometimes, collapsing 2 strollers quickly, carrying strollers on and off...at least with your own vehicle, you are not rushed by others waiting to get on or off the bus).
I do plenty by myself...I take 3 kids everywhere, grocery shopping, to Gymboree activity classes, to the zoo for the day, to the lake for the day, whatever...I have a 6 month old infant, a 2 year old and 3 year old...and we go where we want, with just me. It's safe and fun...
Oh, here's one packing tip: I pack each complete outfit in a zip lock bag (underpants or diaper, socks, top, shorts). I can count the bags for each child and know for sure we have enough changes of clothes. Then in the am, I give the older child one packages - she knows to put on everything in the bag. I give one pacakge to my DH to put on one little one while I dress another. This way there's no searching for matching socks or clean underpants or none of my kids end up dressed in mis-matched clothing combos sadly selected by my DH. And the zip locks come in handy for dsiposing of poopy diapers later!
Go and enjoy -- I'm glad your DH will be ther to help.