24" road bikes are scarce, I would consider a couple of options: Get the MTB, put slicks on it and consider removing the front der and a couple of chainrings. (did that for my DD) Another option would be to purchase the MTB bike and have skinny wheels built up for it. I agree that pushing those kinds of distances would suck on knobbies. You want to make it as much fun as possible, not a chore if you want them to fully participate. One family I rode with on a regular basis had a rule that the youngest set the pace on the longer rides. We used to regularly ride 10-15 mile loops with their son and my daughter leading the pack. At the time they were both riding 20" wheeled BMX bikes with a 5 speed cluster in the back and knobbie tires
(taught me to never underestimate the youngest set!)
Aaron
