I don't know why, but when the eldest 2 started high school mum arranged lifts to and from school with at least 2 other mums between them. If she has the children she drops them off at school, it is kind of on her way to work. I thought at the time it was just until they got used to it. They did not get their first choice high school, so I don't think alternative arrangements were thought of. However you have 6 months to sort this. To this day i don't know why this has carried on, but as she tells us what she does is her business. I believe and it is just my opinion, that they would not get to school on time. They cannot get up in a morning in a timely fashion, everything is always last minute. They did have late marks on their school reports from primary school. Yes she does tell DH, they are a nightmare in the morning and yes they do spend far too long on computers, phones and tablets etc way past what to us is a reasonable time in the evening. But as were told its nothing to do with us. So I think she is making sure they get to school on time and I know people will say this is just how kids are. And I don't think its about using public transport, because just recently as they're getting a bit older they're now asking to meet friends in town etc. And no it's not a dodgy area, there are a few primary schools in the vacinity so lots of children walking to school. Thete is a main road to cross which is fairly busy, but i do believe there are crossings. I most definitely agree that if they can do this they should be able to get themselves to school. I think that have just got used to being ferried around 
Yes everyone is quite right they should be getting themselves to school, and after this thread it's actually highlighted to me that perhaps her requesting the holidays isn't the problem it's how we then deal with it. Because we mostly have contact when DH is off, and this can fall over weekend I suppose we don't really think about it either and what we do is just part of our routine. It's the impact of school runs when DH is at work.
She does not work in a school she is term time but still is entitled to annual holidays as well. However the reason she took a term time job was because at the time they knew they would struggle for childcare during holidays.
@stuffedpeppers actually DH shift pattern is most definitely something she has to think about- why is she being gracious accommodating it ? DH is not in a position to say I'll have children set regular contact on wed, thu, fri for example as he could be working 3 night shifts ?? Or his days off could fall on sat, sun, mon etc. And as she has quite rightly maintained that when SC are here DH should be here too !! So I'm at a loss as to how you think.his shifts are not at all relevant.
Also, no she does not do all school holidays. This Easter they went away abroad in the second week. The first week they were with us Tues-Thu and then came on the Sunday for tea. Feb half term they were here Sun - Tue. However, back to my original point if DH wasnt taking term time hols, then we could have school hols as well. Although as I've already stated I am going to look at changing how they get to school.
They come in the week sometimes for tea, for example DH finishes at 2pm,he might text / ring asking if they want to come for tea, he'll pick them up from school and then take them home later. Their grandma doesn't live far from us and so sometimes if they go there with mum and DH has been on an early, they might call in.
I fully appreciate we have great flexibility but it has to be that way for everyone including their mum.