Sorry, haven't RTFT. However, I don't think the question is that straightforward to answer.
I certainly don't think parents should automatically be given first dibs on holiday over Christmas and New Year and I say that as someone who worked as a Receptionist in Accident and Emergency when my children were small. My shifts were evenings and weekends and, in my 6 years there, only once did Christmas Day fall on my shift. I was offered the shift off by a colleague who didn't have children and was happy to cover my shift for me at double pay with time off in lieu. It was kind of her and I accepted, but would have worked it otherwise. Similarly, my shift fell on Christmas Eve on a number of occasions but I worked those as normal (finishing at 11pm/midnight).
Even with pre-school children, I was happy to book holiday out of the main school holidays, as was my husband. Prices of going anywhere, whether in this country or abroad are significantly cheaper outside of the main school holidays.
However, and this is where things get more complicated - once children are at school and the older they get, the more significant, it is increasingly difficult to be permitted to take them out of school for a holiday. Therefore, parents are restricted to being able to go away on holiday during the school holidays. There were a number of years when my children were younger that my husband didn't get in quick enough and so we were unable to go away, or could have a week at most. What used to frustrate me is that some people (both with and without children) would take three weeks over the course of the summer - with holiday at the beginning and then more at the end of the school holidays. To me, this is not fair but my husband is the sort that never likes to rock the boat.
Nowadays I have a daughter aged 15, so taking her GCSEs next year and my elder children all work. When we go away, it is usually my husband, myself and our two youngest daughters. My husband only has to work round his boss in terms of taking time off, so between them it works out fine.
However, my daughter works in retail and this year, since Easter, she has been unable to have a full week off during any school holidays. She has to work round 4 other people - none of them have children. However one person, who has been in the company for many years, decided to take 3 Mondays off in August (and Monday is the one day that absolutely has to be covered). She then booked a full week's holiday and finally, the last week of the holidays, she was having an operation, so had the next four weeks off.
As a direct result of this, we didn't go away over the summer - costs of going anywhere were extremely high and to not be able to go for a full week made it seem even more expensive. Obviously, this wasn't the case of a parent booking holiday, nor was my daughter a parent looking for holiday but highlights the unfairness of one person being able to book ahead (there are other issues to do with this, which aren't relevant to this thread).
So, whilst I think OP is right that parents shouldn't always be given preference over other employees, in an ideal world, employers would develop a system that is as fair as possible within their company and give and take between employees is also helpful. I did vote YABU because I think it is important for people to be able to have time together as a family - whether going away or not, but I don't think that means parents get overall control. I do agree with others questioning why people without children would want to go away in school holidays, but it's not always that straightforward - some people go away with teachers, who, like pupils, can only go during school holidays and there are others holidaying with friends/family who have children.
Certainly, if I wasn't governed by school holidays, there is no way I would choose to book to go away then - holidays both in this country and abroad are so much cheaper in June/early July or mid September, yet the weather is usually just as good as in the height of the summer.
I also realise that I am only talking about taking holiday for holiday purposes, not for childcare purposes but that is a whole different headache!