Gracie (4 mo old) and Boo (5 yrs) bite and chomp on each others legs all the time. Mainly, I think, because that's the easiest area for Gracie to reach.
Boo, the good boy that he is, never bites hard. Gracie, on the other hand, can go quite "Cujo" on Boo and you'd think she was ripping apart some prey that she has brought down.
But unless one of them is acting terrified or truly biting too hard, I wouldn't worry about it. I've raised horses and young colts will do the exact same thing. Biting and nipping at each others legs.