I'd probably say that most of my early English was learnt by reading subtitles and listening to American cartoons and shows on TV while eating breakfast before school. If it was dubbed, probably once I got my first computer I'd have a way harder time understanding at least the tiny bits I did understand.
I dunno, younger than 5-6 I think most children don't really understand plot lines or whatever anyways so it matters less, a cartoon in English is probably as fun and engaging as one in the native language.
The only reason not to dub a cartoon is that you're an adult who cares about quality and the dubbing is usually done for a smaller audience so it is going to be worse.
There's no reason not to be dubbing cartoons for kids. That's a dorky debate for grown-ass adults playing animu purists.