If your biggest concern is getting back mid day, I'd say NOT to rent a car. The buses are busy at POR (as are most ALL resort transport) at start of day and end of day at closing.
I think having a car is honestly a waste of money better spent elsewhere. By the time you get in, strap up, drive over, park, get the "ride" to/from get in/out of the park, you'd have already been gone by the bus already.
As an aside, if you're looking for a bit of respite while at MK, jump on the monorail, let your little ones have a nap in a stroller (or even on the seat between you) relaxing (and cool). You can always make a pit stop at Poly, Contemp, etc, have a cool drink, relax a bit while your little one sleeps away....there's always a little game room at all the resorts too......just another advantage to staying on site.
People on the boards talk about the buses all the time and I really must say that in ALL our trips (and if you look below you'll see the number of visits) I really had a "long" time maybe twice. We plan and avoid the "mad rush" am/pm, have no issues and POR is an amazing resort.
Whatever you choose, have a great time!