this is what I want to be when I grow up
why not:
this is who etc.?
THANK YOU.
Because you are already a person (who). You will not turn into a different person when you grow up.
You are stating the occupation you want (what).
Hi Saneta,
Both your examples are grammatically acceptable since ‘what’ refers to something inanimate and ‘who’ refers to animate nouns. Logically there could be a problem because instinctively we would imagine the speaker is talking about something rather than a person but at the same time they could be wishing they could become like another person they know or have seen.
Alan
Obviously not in the example Saneta found and was asking about.
There is no ‘obviously’ indicated.
Alan, in primary classes, we are taught to ask questions ‘Who are you?’ (expecting the name as answer) and ‘What are you?’ (expecting the occupation as answer) without mentioning the animate/inanimate concept. Accordingly, ‘This is what I want to be when I grow up’ is the only option.
Still debatable in my mind but then life’s too short to flog it to death.
I’m sorry if you can’t see it, but I think it is obvious from the fact that this is the phrase Saneta asked about, and she asked why the other was not used.
You really don’t need to be patronising do you with your use of ‘sorry’.
If you choose to read it that way then I’m quite sure I won’t convince you otherwise.
Quite frankly, I don’t get the ‘do you’ structure in this sentence. Would you mind clarifying it for me, Alan?
If I can have a go at an explanation… 
It’s a more colloquial word order of a statement followed by a question tag, namely
“You really don’t need to be patronising with your use of ‘sorry’, do you?”
The use of the period at the end of the sentence emphasizes that no confirmation is expected.
Cristina, if what you have explained is what is meant by that word order, I am really thankful to you. After all, you have stepped in to help me haven’t you with the clarification.
Well, there you go!
(Alan might clarify things for us soon enough.)
Although Cristina is correct, I would advise you not to follow that example, Anglophile.
I most usually follow what is more easily intelligible/comprehensible (at the first/second read) to a foreign user of English like me.
In which case, I am happy to hear that you will not follow that example.
Why not in a group of listeners like Alan, Cristina and you?
I don’t consider the original to be accurate, even informally.
At the very least I would say it required further punctuation.
It certainly would not be adapted to similar terms in the way you have given as an example.
You might mean the commas before and after ‘do you’ and the closing question mark. Anyhow, I can’t argue any further on it because the usage is new to me. LUSH. Thanks.