Actually, now that you've refreshed my memory with some more examples of house-elf speech: what's clear is that they don't speak ungrammatically. Rather, they speak their own dialect. Much as people from different regions of the UK (or the US) have their own dialects -- many of which diverge from what's considered "proper" English. That doesn't mean that the people who speak them don't know English well, or don't understand grammar. Southerners who are used to saying "y'all" in informal conversation are perfectly aware that that's not formal grammar, and generally won't use it in writing or when speaking in formal situations.

So when writing house-elf speech, you need to get the basics of their dialect down, and stick with it, rather than just making up random ungrammatical utterances. Likewise, they aren't stupid or child-like in intellect, though they may seem rather simple because of their limited experiences.