The vast majority of the records surrounding the MK ultra program were destroyed, so its truly impossible to know how far it went / who or what was involved. However, I -believe- there's a record out there of some few-dozen of the medical facilities involved in the program. If any one of those facilities was exclusive to children, that would settle it, so if anyone knows where such a list can be found, post it, and we can do a bit of sleuthing. That being said, there was absolutely no shortage of outrageous abuses inflicted on children within hospitals and psych wards in the 50's right into the 70's. A sick and twisted sense of entitlement in regard to mentally ill or developmentally challenged children, often with it's roots in eugenics, led to despicable crimes. Given that climate of abuse within the medical, especially the psychological community in America at the time, it would be hard to imagine a program as widespread as MK ultra and it's affiliates not involving children, though I've neve seen anyndirect proof of it myself.
That being said, I understand there's a fair deal of evidence suggesting Ted Kaczynski, better known as the Unibomber, was involved in MKultra research at Harvard university, when he was only 16.