But they aren’t in public per se are they? Or at school or whatever other silly comparison you wish to make.
They will be in an environment that is completely new to them and they won’t be able to get to their parents and possibly their parents won’t be able to get to them either, depending on the airlines policy regarding walking about from economy to business. Regardless of whether they are seasoned travellers or not, these children may be on an aeroplane that they’ve never been on before.
Plus, is it really fair to expect people who have actually paid, as opposed to getting free flights with air miles to have to put up with kids possibly coming in and out all the time? Or parents doing the same.
Since you don’t have children of that age can you really know what you would do in that situation? And with the best will in the world, children do bicker or tease each other or just generally be silly, more so when it’s more than one child as they egg each other on.
Just because the children have flown before, doesn’t mean they won’t get air sick due to food, or perhaps have mean adults glaring at them or worse, telling them off for going to the toilet too often etc.
@Flymeinbusiness pp have mentioned that the cost of an under-12 bc ticket may well be much cheaper than an adult ticket of the same price. A better bet would be to be with PIL as they check in at the counter and say something to the effect of what a shame they can’t be travelling with you in bc. Perhaps they might get an upgrade or perhaps their own child should consider swapping with one of them!