I think there's several ways of looking at this scenario really depending on your take or personal view. Of course if we're looking at it from a customer service point of view, then I don't think there's a normal person who wouldn't prefer there to be no need for security in this world at all as that would mean the world was a nicer, safer place. Being separated from your family temporarily isn't ideal (unless you dislike them and appreciate the separation

) but unfortunately we don't live in that world and the separation really doesn't need to be that much of an issue for most people. If it's purely a customer service issue for some rather than a safety issue, it seems a bit odd to complain about the request for families to separate on occasion to me? Surely the fact that you're being asked to go through a security check at all has already affected the customer service side of things, being checked before entering has already crossed into the "We want to check that you're not a person trying to take dangerous objects into the park" which you could equally argue isn't the friendliest introduction to Disney either. Nobody seems to have a problem on this forum of security being there and announcing that checks are needed despite that already killing some of the magical vibe before the day even begins. To separate some families temporarily is only really an extension of that, it's not ideal but neither are the security checks either from a strictly customer service standpoint.
For those arguing about the safety aspect of a child being separated and possibly taken by a stranger, how does the rest of the day go? You could argue what happens when say a single father taking his two daughters into the park needs to go into the toilet, or his daughters do? Would we be ok with a man entering the ladies toilet with his daughters, or would we want him to take his daughters into the men's toilets where men stood urinating are in view? Common sense would dictate that most would say "Well obviously in that scenario there would need to be separation" however isn't it worse as the kids are actually fully out of sight? If somebody was that worried about child safety why would they even take their kids to a park and take that risk, you can't really have it both ways.
As others have said if there was an elderly parent who gets distressed about separation for mental health reasons, or a child with learning difficulties then I'm sure a polite but firm "I'm sorry but I can't leave them because of x, y or z" would probably result in your party staying together. If under those circumstances that option was denied then I'm fairly confident that a polite request to speak to a supervisor would be a further and likely successful option.
There's two sides to the coin but I also think that from time to time in life there's going to be rules that we're preferred to adhere to that may seem a pain, but are there for the greater good for the majority. If they disrupt our day a little but aren't genuinely putting anyone into unnecessary danger, then I think it's easier to try to work within those rules unless there's a practical reason why not rather than not because we just don't like it much.