What happens when it comes to Christmas
It seems OP's children are much too special to be involved in "rubbish" regarding Christianity but are definitely allowed to take part in fun things relating to Christ's Mass, which lets be fair - even though Christmas is seen as a pagan festival, it wouldn't be celebrated if Christians hadn't come up with the story of Christmas.
Most people see it as a story. It is, indeed, called the Christmas story.
In exactly the same way that schools used to teach stories from the Bible during assemblies. Assemblies these days concentrate more on love and kindness to others along with basic values and morals we all should live by.
If OP wants to pick and choose which elements of Christianity her children are allowed to take part in then maybe home education is a better option for her children.
It would save unnecessary disruption to the class as teacher tries to figure out whether these precious children should be present for religious teachings, or not. And then have to find somewhere to put them whilst the rest of the class is taught as per curriculum..
Let's face it OP would be the first one moaning if there was an assembly about Easter and her children weren't given a creme egg because teacher excluded them - as per their mothers flaky instruction.