I have 5 kids that have always been great walkers, out of strollers before age 2. They always walked every where.
We returned in September from a 9 day trip. We did not bring one, but not because we did not need one. I have brought single strollers and double strollers. But when kids are 5 and 3, we rent. Only because we can not stand dragging a stroller on and off the buses.
This year was no different are 2 youngest were 6 and 3, we rented a double. It just makes getting around the parks easier and faster. Both of these kids do Six Flags from 10am-6pm, without a stroller, without a Dad to carry them (because I take the 5 boys while dh is at work), I don't carry them, the older boys do not carry them, they will walk the entire park, only get a little whiney on the way back to the car. Other than that nothing. At Disney however, it is a walk around the resorts to the buses, from the bus drop off to the gates, from the gates to the rides, from the rides to the food places, not to mention the walks in lines etc. Not to mention try navigating a 5 and 3 yr old who at about butt level through crowds. Dh thought we could do without, but after literally dragging ds5 around (and he was not whining, just dragging), we rented a stroller for the length of stay. We also passed on our stroller to another family we saw coming into the parks when we were leaving. Pay it forward!
Bring or plan on renting a stroller, it will be the best thing you ever did!
Your 5yr old and your 3yr old will need a stroller, no doubt about it.