Thanks everyone (almost) for the information. That is the type of information I was looking for, that come from people that have been there and done it. I am thinking about all options carefully and hearing from what people have done in the past helps. It is very interesting hearing from people with family far away.
So people understand - the kids visit me whenever possible, so they are frequently with me. They are very comfortable in my care and always excited to see me. We enjoy our times together. Lot's of hugs and love! So no issues there. There can be goodbye tears if they know it may be longer until I see them next time.
We are fortunate with video calling etc. these days that I will ensure they will still be in contact with their Mum regularly while away. My parents were divorced (no video calls back then) and I wasn't keen on my father visits, so have actively made the children feel safe and visits enjoyable over the years so they don't have any of the issues I had.
I will chat with school more and get their feedback. It would make everything easier if they are on board.
Just to reply for comments....
Simonjt - Interesting that yourself and others you know have got exceptional circumstances approval. Sounds like you have done this several times to visit family and not had any issues? This isn't purely a holiday (I know some people will probably not agree) but my family is large and dotted all over a large continent. I am not sure I even recall a time everyone being together like this in the past. So this is the only time we can get to see all family members. If I changed to a different time, seeing all the family just won't be possible. I see schools seem to list weddings etc. as allowed circumstances. The size of this could be compared to a wedding, just missing a bride and groom! ;)
Pythonesque - Thank you for your input and advice - your situation sounds similar. I think 3 weeks is the minimum. I looked at 3, but calendar dates don't really work out for that. You end up travelling in the wrong time of the school week so it's either straight back to school or lost days until starting the following week. A fine balance of getting the most days of trip to avoid jetlag and the least days from school at the same time. Let's not forget about the lost days in the travel itself. I welcome your comments about heading away from UK summer to winter over there. We will be south, so possibly the wettest and coldest time of the year. The local kids (the cousins) will all be in school. We would basically travel to the other side of the world to be stuck indoors. Ok, I would get to see some of my family which is always lovely but it makes it pretty dull for the kids. I know they would soon become bored. It would be like going to the south of France or Italy in December instead of July. You are correct in saying that this could be the last chance until they finish school now. Absolutely agree with you on jetlag. It is tough, and very much so going there compared to coming back. Maybe because the trip over is '2-3 days' going by the calendar that direction. I would love to get agreement in writing, and this is something I will aim for.
McKenzieFriend001 - This is a worry. Was your friend going to visit immediate family or was it more of a holiday? If it was a family visit, were they able to see all the family regardless of the time they went? That's very sad what happened to your friend. This I why I think need something in writing.
Fooksticks - I wish we could do in two weeks like you managed but we won't be just staying at home like you were able. It will be a busy time while there. The kids already have a list of places and stuff they want to do. I did two weeks (and I was much younger too) and it was totally exhausting even though it was a quieter trip. I said I would never do again. I thought if you didn't get approval from school, they would fine. So it's interesting to hear what happened to yourself and not being fined?
NBLarsen - Thanks for your kind words. I can't help but think if I was a mum doing the same thing I would get these comments? But the majority are helpful. As I mention above, kids are very happy in my care. I have planned to include school work and suggested this to the school. I did ask if they could set some work but they said it's something they don't do. But we already have a good online learning tool we have used for extra studies that we can use anywhere anytime. And of course the ideal is both mum and school are on board. People sharing their experiences and information can only help to achieve this.