I was referring to where the -n came from. After research it seems it came from the old case system, which originally came from german, not sure if it's the dative plural, was just a theory. Here's more about that:

https://en.wiktionary.org/wiki/-en#Etymology_2

Read the article; the -en in this particular case arose after the loss of the case system, in a hypercorrection during a period when -en and -s were the two main plural endings.

I did, I was speaking about -en vs -n. Why -en for children and not just -n. Have I missed something?

Plain -n isn't used anywhere in English that I know of for a plural; the consonant cluster childrn would be improbable.

It doesn't come from German; both English and German inherit it from Proto-Germanic.