How far can i run safely?

Don't do it too much. Our bodies at 14 aren't fully developed and running 10 miles every day is harmful. And mixing it up is better anyway. If you want better times do this. Day 1. Hard 3-5 miles. Day 2. Sprints 10x200m with a 200m walk in between. Day 3. Cross Train (Bike, Swim, Lift Weights, do stadium steps, but DON'T RUN! Day 4. Medium 3-5 miles. Day 5. Easy 3 miles. Day 6. 8-11 miles hard. Day 7. Rest. Maybe an easy 1/2 mile jog. Repeat

If not for time just take more days to only 3-5 miles

I'm an orthorunner witch means my body doesn't have a distance limit....Just poked my self in the eye. When i was 13-14 i ran 13.5 miles everyday except a few for those two years. It never tired me. there were no cons to the running. It's also how i got my first phone. What my mom like to point out is that i did this by myself.

It depends if you have built up your mileage.

If you have been building up to running 10 miles and do not do it everyday then it is fine.

Just if you went from running 2 miles to 10 in a weeks time, that could be bad

Start off with small 15 minute runs/jogs, and in the event that they love it, you'll be able to begin slowly making the runs longer. If you're going to take them for runs, watch them. If they begin to seem worn out or do not desire to run, sluggish down.