M.H.
I disagree with the renting a stroller. A double is actually $30 a day, and since you only live an hour plus away and are driving, bring your own. I do think a double would be best as your 5 year old is going to be exhausted by the end of the day.
Pack what you want in your bag and plan on leaving it in the stroller when you go to the rides, etc. Disney is pretty good about keeping them safe and we have never in the 20 plus times we've gone had any issues with things being stolen.
If you are staying on property, I would suggest looking in to the meal plan as overall it will be more cost effective for you, but if you aren't, you are on the right track. You can bring whole lunches in to the park (which would save you even more money) by bringing in a cooler, so it's something to think about.
Every park has something for your guys. We went when my son was 2, 3, and now 4, and each year we find new things that he loves. Hollywood Studios (what used to be MGM, it slips me up too!) has tons of Toy Story stuff, so that would definitely be great for your 3 year old! Every park has benefits though (even Epcot), so don't rule any out!
My biggest suggestion is to run your day this way:
-get up early and get to park when it opens
-stay until lunch time
-leave after lunch for pool/rest time
-go back to the parks in late afternoon/early evening
-stay until everyone is pooped!
You will find that the recharge in the middle of the day (which is also the hottest part of the day!) will help everyone feel better, and you will be able to do more at nighttime when it is cooler!
If you have any questions, please feel free to PM me. We go every year and have tons of great info! :) Have fun!