Off with alison56's head!
Why? Really?
I don't get that. I use simplified sentence structure with my DS, but I don't use drastically different words to the ones he'll hear in real life.
Do adults use "ta" where you live? If not, why use a word which has no meaning in "RL" as we say on here? Because "ta" doesn't really mean "thanks" to a toddler, it is usually used in response to him giving you an object so in his / her world it just means that the adult is pleased.
The concept of thanking somebody or being thanked is way beyond the comprehension of little children, indeed it's apparently too much to ask from the adults living in the depths of the council estate near me.
All toddlers experience in response to their actions is approval or not, and they don't often care, that's what differentiates them from school children who usually aim to please.
They are egocentric and have to be guided and "ta" is a positive response in the same way that, "good boy", "kind girl" would be. It doesn't mean that they appreciate and comprehend the social arrangement that constitutes thankfulness, so why decide that "ta" means something is clearly doesn't?
It's a nice response, and a meaningful one in terms of communication, but it's one of those things where the concept comes much later, as any person with young DC would know, apart from the professionals it seems.