The past heated discussion aside, surely part of the problem here is actually defining what is 'religious', and thinking about that from both a young learner's and an adult's point of view.
Schools do all kinds of things which are 'christian', although it's often non-christian faith groups that notice-they work Monday-Friday, serve hot cross buns, observe Easter and Christmas holidays etc. They are also compelled to provide RE, even if the framework varies. And you can find one of those OK and the other not (I do).
Whether broader Christian practice, such as the discussion of abstinence or self-abnegation prompted by Lent should inform is part of this issue (since Cosmo and Glamour do this too, as might an atheist KS coordinator). Because, after all, the OP's DC did NOT understand this obligation in religious terms, he didn't know why (and a real god bothering would have made it clear).
Or is the objection instead to an imported religious figure? In which case I have sympathy, but equally see children exposed to quite cool puppet shows giving cultural references to bible stories, and then asking questions ranging from 'why did god flood the earth?', 'Why was he mean?' to 'where did the snakes live' and 'how did he find the chameleons'?
I can see why many parents, of both non-christian faith and atheist backgrounds are worried by the creation of a 'space' where dogma can exist, of room for the acceptance of a particular God. But the question is surely whether this happens not if we can see that logically it could-after all entire curriculum allows room for power to assert itself-history for nationalist preaching, geography for westerncentric theories of development, English for RP over dialect etc.
Basically, if you want to change it, you need to be clearer and more precise about what you are complaining about: 'religion' is a broad and useless term.
Basically, I can see the worry, but I think children rather more resistant to indoctrination than many here, and certainly resistant enough to the rather weak strain thrown at them by puppet shows and vicarly chats.