If he likes both then good for him, but like most I think he will eventially have to gravitate towards one. If the juijitsu is anything worth while he'll find that his body will suffer and the rigors of taking two martial arts at the same time will have its toll. Also if the kids are just rolling around and learning escapes I think that is fine but if the teacher starts to teach them chokes, like triangle chokes, and arms bars I would pull my kid. I don't think kids have the control to know when to stop. I like to see kids in Judo anyway. Most kids go to the ground when they fight and Judo can give that knowledge as well as throwing without bringing in the things like choking and arm locks and wrist locks. I think its just safer but you can only work with what is available to you in your area. I would just definitely monitor the juijitsu.
It is up to the instructor to convey the limit.
That is 2fold: The person in submission has to tap out, the other person has to stop NOW! And while kids get a little carried away in the non TKD programs, when safety is at stake, the fun stops yesterday. And yes, I got after a youngster for performing falls in a wrong and unsafe manner...making little kids cry is just all in a day's work for me (that his daddy told him not to listen to them there white women is another story in itself and subject for another day)