Assemblies are not necessarily about 'Christian' worship.
But these are, @BlueSlipperSocks.
And even if it is all about Christian worship in OP's children's school what exactly is the problem with them singing a song - and there are never hymns anymore - or learning about ethics and values?
Come off it, you know that a religious ceremony will involve more than singing songs. It will involve prayers, and talking about the Christian God and Christian teaching. If they are talking about ethics and values, it will be in that context.
And I'm sure you don't believe that the only place children can learn about ethics and values is via Christian assemblies.
Are your children exempt from taking part in the largest Christian festival every year OP? Are they exempt from learning about The Nativity? Or are they only allowed to take part in Christmas plays if it involved Jack Frost? Do they have Easter eggs?
Not that one again. Christmas and Easter originated as pagan celebrations, it is perfectly possible to celebrate them without taking part in Christian acts of worship. Since OP is fine with her children attending RE, obviously they aren't exempt from learning about the Nativity; the point is, they don't have to believe that it has anything to do with the child in question being a son of God and all the rest of it.