JRT's are high strung and often tend to bite. Sure they look cute when they strike the RCA dog pose but mine require nearly as much attention as 3 year old children.
Beardies will 'nip', part of their herding instinct, but they are not an aggressive breed. We had another when I was young, and she would herd cats, rabbits, and us kids if we were all running around in the backyard. I'll never forget her biting a neighbor of mine square in the butt-cheek as we were running around my backyard. My Mom still laughs about it, she never liked the kid anyways!
My JR seems to be the exception to the little kid rule probably because we had him nuetered I'd imagine. It's pretty neat though, Chaos (thats his name) pretty much adopted our now one year old as his boy. Chaos won't let any non family member get too close to him without a warning bark. They've been great playmates and we keep it well supervised