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.
I think you should also bear in mind that there could well be a hierachy amongst groups of house-elves working together. For instance at Hogwarts there would have to be at least one house-elf who was in charge otherwise none of the meals would be prepared on time. They know their place, but they'd need someone to tell them who does what and there's no evidence of a head wizard/witch chef in the kitchens.
Kreacher certainly seems to be in charge at the Battle when he's urging all the other house elves on.
Also the Hogwarts house-elves treat Dobby and Winky with disdain and fear almost - as if their free elfness is catching. I think their speech towards free elves would be slightly different. (Look at Kreacher and Dobby's interaction as well)