If they come with advice to use only above 60 lbs, it seems they did find out beforehand. With that weight limit, the 12 year old girl is probably a non-issue, unless she's severely underweight.
The boy, however, was likely under it, and so it should be investigated. I would hope the officers receive training including knowing that limit. If this is a case of one officer ignoring procedure, he should be dealt with. (Remembering, of course, that he took a calculated risk to save the child from what he saw as more serious harm.) If this is a case of the MPD not fully informing its officers in the use of their equipment, then they need to shape up.
Either way, the woman should be far more concerned about her kids desire to cut himself than his tummy ache from the bad cops and their stun guns.