3 is still pretty young to walk an entire park daily, even with a break! I have an extremely social, energetic 3 year old myself, but I know that at some point in the day she'd want a stroller, and I am certainly not carrying her through the parks! Personally, I think you may be setting yourself up for a tantrum and/or a LONG walk to Guest Services to reserve one after a long day. Plus, it's less expensive to rent outside of WDW. I'd go ahead and rent the stroller, Magic Strollers is great, they have free delivery to the resorts, and you can even get a double in case the 5 year old gets too tired as well. In 'real' life I wouldn't have a 4 or 5 year old in a stroller, but WDW is a totally different story. When we go next Dec. our kids will be 4 1/2 and 14 months, and we're bringing our double stroller. Have a great time whatever you do!