I started with mine at the age of 5.....I thought it was a bit young and then realised from my Strava that they had walked 11.3 miles at Alton Towers in one day without complaining once as they were enjoying themselves and distracted!
But: they don't do slow and steady, they run ahead, climb banks, skip hop and jump...until they are tired and then instantly want to go home. So you have to factor that in to things.
I would do, for example 45 mins walking, 15 minute break sat down with snack, 45 mins walking.,,,,longer break for lunch, 45 mins walking, 15 min break with snack, 45 mins walking finish.
My theory (not based on any science I have ever heard) is that 45 mins is about the longest they go without crashing and needing more fuel. If you factor this in and literally stop every 45 mins and feed them, they will go all day. But if you try and push through for more than that without a break, they will get beyond carrying on.