can they now? It's not the job of the receptionists to babysit and they have other things to do
It is their job if the school chooses to make it part of their job description, Don Corleone. It's one of a number of possible methods of complying with their legal obligations. If that is how the school chooses to meet legal requirements, it's not up to staff members to object.
Who is going to fund the extra hours of the play or lunchtime assistants? you are not suggesting they work for free do you?
Why is it extra hours? A TA sits outside with a few children rather than sitting in the assembly hall with a number of other children. S/he is being paid either way.
If there is an extra cost, the school has to deal with it, just as it has to deal with the costs involved in complying with all its other legal obligations. It's precisely what the school is given public money for. I simply don't understand your perception that it is somehow totally unreasonable to expect schools to obey the law.
One parent doesn't want an assembly, one parent doesn't want PE ...
The difference is obvious. There is a legal right to withdraw your child from acts of worship, there is no legal right to withdraw your child from other aspects of the curriculum. Absurd argument.
Worship in one thing, but hearing about a subject you don't like is another.
Yet again, there is a basic difference between RE and acts of worship. OP has made it clear she is perfectly happy for her children to learn about religions. Why do you keep ignoring that?