Peachey: I see what you are saying. I had said no at first, but then thought about it a little more. I was trying to look at both sides (which I think is good practice when answering questions)
The only consideration is that, like we do with animals (meow, etc.), I have also heard people mimicking babies with "ma ma" and "da da" and "pa pa" as the inherent "sounds" a baby would make.
Although this would probably never happen, if someone were to ask me what sound a baby makes (just as what sound a clock makes, what sound a dog makes, what sound a drum makes, etc.), probably one of the first "sounds" I would think of is "ma ma" or "da da," or even "waaah!" (for a crying baby)
It is not NORMALLY used in an onomatopoeic context (and, in an effort to not deceive Victor, I already clarified this in my previous answer), but I would not say a definite "no."
If we can mimic animal sounds, why can we not mimic "baby" sounds? Wouldn't both technically be onomatopoeia?
So, I suppose the questions now are both:
1. *IS* "ma ma" and "pa pa" onomatopoeia?
2. *CAN* "ma ma" and "pa pa" be onomatopoeia?
My answer:
1. No, not usually. Most times "mama" and "papa" are names a child would call a parent. This would DEFINITELY not be onomatopoeia.
2. Yes, given the context of someone mimicking the sound a baby makes. In my mind, if you can call it a "sound," then it technically qualifies as onomatopoeia. (I suppose this depends on the perspective, though. Probably most people would think of a habitual crying sound first before anything else.)
Source: myself (American English)