I agree with Olin Lathrop, that distance seems too long.
In my experience, a base number for children is appx 0.5 mile per age in years, up to around 10 years old. After that, figure about 1 mile per age in years.
That's roughly 1km per year, up to 10. Maybe 2km per year after 10. Hoping someone can come up with easier to remember numbers than mine, my metric is not very good, sorry. :-(
So a 5 year old should do no more than 2.5 miles. Can they do more? Sure, absolutely, if they hike often, play sports, and otherwise are active. Less if they are couch potatoes. Will they want to stop often? Yep. It's just a baseline.
Consider the climate, weather, your activities, their equipment, their dress, their experience, bringing (their) friends, meals/snacks, terrain, and sights along the way. For a 5 year old, don't just plan on many stops, but do plan to do things that are fun.
When you factor in terrain, poor weather, higher or lower than comfortable temperatures, higher elevations, you might want to move that .5 to about .3 for mildly uncomfortable effects, and a .2 for moderately uncomfortable effects. So, a steep climb, move it to a .4. Very hot out (or cold)? Another notch to .3.
I think the "messier" the trail - rocks, puddles, streams, etc - will make the trek take longer, and maybe less desirable for the adults - but definitely more fun for the kids.