I am a practising Christian, married in the CofE, keen to send my children to a CofE school. I also thought a class wedding, in church, was a bit of a weird idea for 4-5 y.o.s.
But it wasn't, largely because they didn't actually act out a real wedding. They can't, in fact. Marriage vows are performative, they do the thing the words describe (as long as the right conditions are met, which obviously they weren't either as 5 y.o.s can't legally marry), and priests are very aware that they need to be really careful to make sure that form of words is only used in the right context and people don't accidentally marry when they're not supposed to. The vicar talked the children through the CofE wedding service and paraphrased it for them, so it went something like this:
Vicar: "Before two people can get married, it's my job to check that they actually want to marry each other. And then, when we're sure they do, we get to the bit where they promise to love each other and look after each other for the rest of their lives. So Josiah, you're pretending to be the groom, you say, 'Georgia, I promise to love you and look after you for the rest of my life', and then you can whisper 'Not really!', because we know you're only five and this is only a pretend wedding."
So it was learning about a sacrament in the CofE, yes (and there were other occasions, like the Chinese New Year lunch, when they learned about other cultures and belief systems). But even though they were dressed up, it was very clearly not a creepy enactment of an adult rite of passage.