just an idea....here in north cumbria there's a grouping of clubs called "team north cumbria" which draws juniors from three or four local clubs (carlisle aspatria, eden runners etc) so that together they have the critical mass to compete with bigger clubs in the northwest in the athletics (ie track and field) league.

you could use a similar approach if you thought one of the problems with the junior fell relay idea was a lack of numbers in any one club.

the problem I see with junior relays is that if have to scratch around to make up a team, many of the 3rd or 4th stage runners can end up running the whole of their stage on their own rather than competing in a pack with others. This isn't much fun for them, especially if their team is coming last or nearly last.

In my experience team events rather than relays work better for juniors.