I've been twice with my family and my nieces. The first trip my youngest niece was 3 1/2 and the older 6. The next trip they were 5 and 8. Both times both girls wanted to walk but little legs do get tired- especially for the younger one. My DH and I have decided for our upcoming trip to let our DD who will be 4 1/2 walk in the mornings and rent a stroller after lunch. She is very independent and won't want to ride in the stroller - especially with her older cousins walking- but this is something we are going to talk with her about before we go and stick to. Our last trip our youngest niece who was 5 walked the entire time. By the end of the week, she was exhausted and miserable- not a very happy girl! We'll see how this plan works. It will allow her to walk some, but still save her energy for the rest of the week. Also, by the end of the trip we may just rent a stroller in the morning for the day and not let her walk- we'll see how things go and play that one by ear.
We've also found that renting a stroller is much easier- especially on- site with the buses. We took our own last time for our 6 month DS and will enjoy the freedom of renting. This is just each family's personal choice and preference.
Have a wonderful time and enjoy the trip!