One very good reason, really - sexual predators who like young children ...& teens for that matter.
We used a sleep-over for our young daughter once, in an effort to help her break the ice, so to speak at a small International School in the EU, she'd started to attend.
We invited the parents to reassure them that we'd be present throughout the event - yet the fathers didn't even bother to show up with the mothers, either because they didn't care, the invitation omitted to mention drinks were on offer, couldn't be bothered, preferred to watch the telly -all of the above - as no reasons were given for their allegedly "logical" absence on a Saturday evening.
To say the experience for us was an eye-opener, is no exaggeration. The TV room had been done up with big cushions & they (all girls aged 8) could sleep there together should they eventually choose to do so. An abundance of snacks was available in the kitchen & we exited Stage Right to a sitting room up stairs.
After the sun had set, some games played under my watchful, but reasonably distanced eye & they'd all been fed & watered, one little minx decided it would be fun to play the "Let's have sex!" game. In the end, this game amounted to who could emulate certain 'movements' & noises for the loudest & the longest. Fortunately only 2 "performed" while the rest laughed uproariously. It could have been worse - sex toys might easily have entered the picture.
Our daughter's father sharply decided to disappear upstairs into his office & lock the door, advising me to let him know when I planned to retire, safely, for the night.
This event taught us that one can never really know what goes on in other people's homes, let alone in one's own extended family.
I let our daughter attend a sleep-over at a classmate's home a few years later, where I knew the family had no male relatives lurking in the background & whose father would be away on a business trip. Of course, there was no way of knowing whether this family watched porn regularly or left doors open during their possibly noisy sex, but by then our child had learnt about these issues anyway.
Yet it's astonishing how clueless many parents still seem to be in assuming that other couples in the neighbourhood are "like us" in most ways, simply because their children attend the same schools & religious institutions & the parents seem to ascribe to the same political outlooks. But truthfully, when it comes to sex, preferences are not at all as "obvious" nor "the same" as political or economic leanings say, can be, so it's thus dangerous to assume that they are.
Nor is sexual deviancy, in particular, a subject that comes up at the school gates, regularly - well not in my experience, anyway. Of course the reason for reticence at the school gates might very well be because deviant sex is quite common, as The Telegraph's agony aunt & uncle inform readers occasionally, but demanding as it does, the involvement of select groups only & rather hypocritically, a certain degree of secrecy about its nature & who participates, in an effort to protect their own children/spouses from any shame or embarrassment, but not yours, of course.
So it was with relief that the age passed swiftly for us, when, for some mysterious reason, many mothers, aping their American sisters, seem to think sleep-overs are yet another great idea & a way to show off their homes & entertainment skills or play the role of a "Real" housewife for a day.
The time is now, when we should return to celebrating something as special as our children's birthdays within our own families only, while the child is under-age at any rate, instead of using the day as an opportunity to show children "how much" they are "loved", while simultaneously playing games of one-"up(mother)ship" by the sheer numbers of "friends" invited - something that also lets parents off the hook for having to expend more energy & time with their child than they want to. The big crowd also shows the neighbours how popular the child must be. Some parents might use their money to up the ante by hosting the whole "Year".
It's actually quite surprising how few parents, rather stupidly, are unaware of the dangers of locking up a group of young, vulnerable, under-age children in one's home, for the night. The bigger the house, the more suspicious I became about parents' motives - especially if they never bothered to greet me at the school gate, during the rest of the year.
It's not only sexual predation that might be a problem, but also certain children in the group, who could be bullying one of the others, who then becomes trapped. Like so many teachers in the not too distant past, or even the present, a parent "on duty" at the event, might be utterly clueless as to how to deal with the fall-out & the consequences of bullying, the memory of which can remain for years in the mind of the aggrieved child.