When she was 7, I got for my daughter a 10 year old MTB. Red (proper colour helps), without suspension (it saves about 10 lb and lots of technical problems), 12 speeds and aluminium rims (better braking when wet). She got a funny animal for a horn, so that was a great morale booster. I also asked the bike shop to replace the original tires with narrower slicks, which means less resistance and less noise. I also added fenders, a rear rack (just like dad) and lights.
As I looked at my parts bin, I decided to change the original 1x-28 freewheel for a 12-32 7-speed freewheel I had, so she would have an easier climbing gear. Right now at 9 years old, she is about to outgrow her bike, which now has a 400-mm seatpost, high drop bars (narrow 20-year-old bars) and bar-end shifters in friction mode.
As for distance, at 7, she was able to ride 20-25 km on her own, and now she is able to ride 40-70 km on her own (as opposed to stoking the tandem), depending on weather, traffic. Since we live in a not too hilly environment, the most challenging aspect is actually finding routes that are not too hilly. She doesn't have any problems climbing hills (up to 4-6%, I guess), it's going down that she finds a bit frightening. It's only this year that she has started to go downhill at more than a crawl speed.