You work every day (depending upon contract) that the school is open and often 5/10/15 days extra in the holidays and some evenings/possible weekend things.
As a result, your holiday entitlement is taken when the school is closed. However, the TTO contract doesn't pay you for a number of weeks where the school is closed, as the leave entitlement doesn't stretch to take account of all the closure days - it represents something around a 20% deduction in salary, so somebody with a 5 days a week fulltime TTO contract is paid the equivalent of working 4 days a week all year round.
For every hour(ish) a day reduced in those TTO work patterns, you then lose another 20%. So, for example, if you worked 30 hours over 5 days TTO, you're paid the equivalent of somebody working 3 days a week all year round.
And then the actual salary that you would earn in another job is technically reduced each month because it's paid over 12 months rather than the 10 you're actually working - as though your employer said 'Well, you've earned £1200 this September, I'll give you some of it now, but I'm going to hold on £100 to give back to you next August when I won't be paying you otherwise because I don't want you to work then apart from the weeks where I do because it's better/easier for me and you most likely wouldn't be able to cope with the concept of not being paid at the end of August if I didn't do you this favour'
It therefore makes it even more restrictive, as you can still end up having to find childcare for the entire summer, depending upon when they want the + days - they could say every other day (meaning you don't get a holiday at all), end of July, middle of august and end of August (so no more than a week at a time), two-three weeks in August (leaving you the last week of July and first of August for holiday and your spouse also having to try and get time off on that fortnight because it's the only one, especially as some childcare provision - holiday or childminder - will also be unavailable due to holidays in August) and you'll be in for training days before your own kids are back at school come September 1st.
The only advantage is where your children's school dates exactly coincide with your school dates, including early finishes, delayed start times and whilst your training days never match with theirs, their 5 training days are covered in their entirety by somebody else (husband?) and your childcare only takes leave exactly when you aren't required to be in work. Oh, and you don't need to attend Christmas plays, sports days, prize givings, see them off on trips, collect them early or attend parents evenings.
But if you're on the bones of your arse and don't have relatives/a spouse/ex to look after your children, having compulsory time off between Christmas Eve and New Year's Day can be useful, as you're not in the position of having to argue that you genuinely cannot get childcare for 27-31 December each year when others are saying 'well, I don't have children, why should I have to work on those days just because you do?'.