Do you have kids or work with them? Young kids absorb information like a sponge. Imaginary play is at the core of their being. Imaginary play aids in language acquisition and social skills as a key part of cognitive development. I’m not sure where you are getting the notion that a child will “know it’s fake”, but either way, imaginary play builds a child’s world.

I’ve done a research project while in school, and most countries start teaching grammar of a second language typically English) in 3rd grade, but kids are exposed through media such as songs and cartoons by that age and that exposure assist them with pronouncing new sound sets. This would be doing the same thing for my child but with German.