ONTpax and others put it well. Having thought about it some I agree. As a passenger with a child, I say ban UMs on aircraft (and trains!). As ONTpax says, these are not day care centers, and you all are travel and safety professionals, not day care workers.
Safety to the UMs, yes. But what about safety to other passengers? Do those emergency plans you study and practice as crew take into account the presence of UMs? I read about these FAA evacuation experiments, where manufacturers try to show they can evacuate the plane in so-many seconds. Do those experiments, or your crew practice drills, include four crying UMs frozen to their seat in fear? My guess is no.