I've definitely heard kinder used in a normal English setting to mean young child. It's as much of an English word as any other word we use in everyday life which is borrowed directly from another language.
I've never heard it used that way. Well, one major exception -- Kinder Surprise eggs. There are definitely other directly-borrowed words that are arguably "more" of an English word now. Ballet, antique, aficionado, doppelganger...
I mean, those words are just more commonly used. That doesn't make them more of an English word.
I think somebody else LF mentioned it already, but "kinder" meaning "child" isn't common in dictionaries either. If you search on dictionary.com, for example, you only get the definitions for "kind".
Edit: I guess you could argue that being English or not is just a binary value, but I don't think it's a good argument. Language is fuzzy, and there isn't one overriding authority that determines what is or is not a word in a language. Usage is the driving force.