What age should juniors start

My 9 yr old son and 8 yr old daughter are both keen to start running with me.There seems to be a lot of conflicting opinion as to what age kids should start running and how much mileage they should do. Can people please suggest what sort of running , if any I should allow them to do.

Any sort and as far as they are comfortable with, as long as it is off-road and properly supervised. X-country and trail running would be good.Hard surfaces and young growing bones are not a happy mix.
I am worried by the large groups of youngsters I see running along ill-lit roads on dark evenings. OK, so they are accompanied by adults and have reflective vests, but there are some very dodgy drivers out there and, as stated above, a lot of running on paving\tarmac can do harm.

I belong to a running club with a large number of juniors. They ususally run on the track or in the summer off road. In the winter we have to go on tarmac, but its usually paths. The youngest ones (age 6 - 9) usually do about 3 intervals of around 300m and then do loads of 'fun' stuff. I have a son who is 9 and on Saturday he completed his first XC race. They have to be 9 to compete. The maximum distance that age group (U11) can race is 1,600m (a mile) He has run about a mile and a half, or up to 15 minutes. The coaches are not keen on continuous running at that age, prefering intervals. I find that even at 9 he can out sprint me! My advice is to find a club if there is one in your area and ask if they cater for juniors. Not all clubs do. Dont worry that a club is for especially talented athletes, if it is a good club they will welcome any standard of junior or senior - if they dont, then walk away.

Thanks for your advice both of you.We live fairly close to Colchester who have a good athletics club.I'll contact them. I have a good treadmill at home as well but I was unsure as to how far I should allow them to run.I think maybe limiting them to no more than a mile and a half would be sensible.