Thank you to all the US parents who gave their opinion/shared their experience, all very helpful. Thank you also to the non-US parents who gave thoughtful responses.
To those who just waded in with their shock and disapproval – do you really have nothing better to do?? You could really do with something to get you away from the talk forums. You know, maybe the UK could do with a secular family festival in late November… 
Right, just to be clear, it’s not a consideration, we are taking him out of school. The school doesn’t fine for one day of absence.
I really just wanted to hear from others to see if they do take their children out and, if so, if they tell the school or if they just have children who are conveniently unwell every 4th Thursday in November. A US family in DCs school do take the latter approach and it seems to be a bit of an open secret, maybe we should follow their lead next year. As it is, we have told the school and he will be going back in to school on the Friday with a selection of his Thanksgiving crafts for a bit of a show and tell.
DH doesn’t ask for much in terms of supporting cultural ties to the US other than our trips there, just Thanksgiving as it’s so important to him. The other Americans in his office also take the days off although they don’t have children. As BeautifulWinter said, the way I look at it is if we were living in a country where the 25th December was a regular work/school day, if it was at all possible, I would absolutely take the day off and the children out of school.
Please don’t feel the need to respond again if you are not a US parent or if you don’t have anything genuinely helpful to add. Really, go on, do something else. Go and make a cup of tea. Treat yourself.
Peace.