So, if finances are a concern and you would rather keep it to one day of park tickets, then I'd suggest doing the park day in the middle of the trip. The other two days I'm assuming you'll be flying in and flying out. You may not have access to your room when you get there, but they will absolutely let you use the pool facilities and such. I'd have a carry on with swimming gear and fresh clothes for afterward as you won't have access to your luggage if you let Magical Express handle it (which I prefer because I've only got so many arms as a single mom!). Then you'll be able to go to MK first thing on your full day there and take advantage of any Extra Magic Hours that Magic Kingdom may be running and be able to go in early when the crowds are less, rest in the middle of the day, and go back to enjoy MK in the golden hour and how magical it is at night. You will NOT be able to do everything, and particularly with younger riders, you may want to prep what you want to get accomplished in advance or you'll waste a lot of your day wandering around. I have a whole theory on how to break up MK with littler legs if you'd like that info. I'd also suggest staying on the Skyliner, less expensively would be Pop Century, or there is Caribbean Beach at the moderate level. Pop's pools are wet and fine, but Caribbean's has a couple of water slides, one for smaller riders and one for the slightly more adventurous. The Skyliner though, is a BEAUTIFUL ride that spins you to Epcot and Hollywood Studios, and is relaxing and entertaining and free. I recommend it at sunset because, STUNNING. It won't help you travel to MK, so you'll use the bus for that, but just having that other thing to go and do that is stress free and relaxing is awesome. Also, you could get off at the Epcot stop and walk over to the Boardwalk Resort, which is a LOT of fun too.
If cost is not the concern, and you're just looking to not overwhelm any of you, I'd recommend adding one more day to your trip. That gives you two days at MK, which I build in to any trip I take, even traveling on adult-only trips. It really is that big, and breaking it up into two days means you feel less rushed.
Happy travels!!!