Occasionally things happen because a combination of factors. And yes, human error is one and it's not always a cop out. I made a mistake at work once which shouldn't have happened normally. My dad was dying in hospital and I was under a lot of personal stress. I perhaps shouldn't have been in work but I'd thought I was ok, and it was a school, so no-one to cover me. However, that particular day work was particularly busy and more stressful than most days, with other staff off, lots going on and just less time to think actions through. So I was rushing through my tasks, one of which was to do with medication, had what you'd call a "brain fart", I suppose, mind on my dad, and made the error. It was all ok in the end but I was really upset. There was no lack of knowledge, no lack of competence. I don't know how you mitigate for staff members who are under personal stress but don't realise it. It's not like I work somewhere where we are adequately resourced, with enough staffing because it's education and that has been under resourced for a decade or more.
But do we say to parents that because of that small risk of multiple factors coming together on that particular day, involving human error purely due to staff personal stress, that we will not give medicine? In many schools there simply are not multiple staff available at a particular time to take over a task that would ordinarily be done by someone different.
OP, I would actually be specifically asking the school what went wrong with the evening medication-giving process? As clearly the morning process worked fine , so what was different about the evening routine? Was it that the time that you had specifically requested it be given at a time that was difficult for staff to manage because they had other/too many responsibilties at that time, or because there was a change of plan in terms of activities and so the group was not near the place where that particular medication was kept, a mental plan was made to give it later on and then it was forgotten about?