My club, the Hitchin Nomads
, was involved in this year's Muddy Monsters
series - kids' cyclo-cross. That site looks only partly developed, but this Google search
returns images - the ones of kids are mostly from our event, I think. Should give you the idea.
We've also been involved in trying to get some coaching off the ground on the RAF station at Henlow (wrong side of Hitchin for you). Don't think it's running at this moment, but I could ask.
There's also the Welwyn Wheelers who should be good for juniors. There's an outdoor velodrome
at Welwyn where various clubs do events from time to time.
There's a mountain biking facility at Rowney Warren
near Shefford. In broad terms, the nearer you go to the middle of the wood, the more challenging it gets. Round the outside is fine for family rides.
I'm hazy about what Stevenage CTC
do for young riders, but they seem to have some, judging by their site. They are pretty active.
PM me if I can help more.