I am not sure why you would ask this question in a support group ? The max target heart rate is 220 minus the age That is the MAX. You should only exercise between 50% and 75% of the max. I would hope all the children had a complete cardio physical before you done this. Another thing to keep in mind is the recovery time of the heart rate.
My question to you is did you have a AED while doing this or medic`s with a defibrillator ?
I use to be involved with youth sports and had to send a twelve year old to the hospital with to high of rate. Turned out the child needed an ablation, He had passed two physicals before this. I am no expert by any means but I have HCM and know what can happen. You can never be to cautious working with children they do not always speak up when having symptoms or may not even know that they are having symptoms.
I don't know but the major question in my eyes would be why would you unless you are involved with a confirmed and accepted clinical study of some sort which i am sure must have been done by now....to me it would seem a little dangerous without the proper supervision of each and every child and the correct equipment needed if one of them went into distress? Establishing clinical relevence to me for personal revelation could be a little outside of the boundries........i know if i were their parents i would probably have an issue with this one.....i would advise caution and in looking for a correct stratagem....well the word alone ...................................