Look, you are posing these same hypothetical questions on a different forum. Isn't it pretty clear to you that if the parents wish to leave him with some friends or relatives while they are deported, they are free to do so? There is no need to get the government involved. So, what is the purpose of you asking these irrelevant questions? The US is not deporting the US citizen child. He would go with them if they make no arrangements for his stay with anyone else.
By the way, if the parents are deported, then there is a high likelihood that they will not be able to return to the US even once the child turns 21 and sponsors them if they will be barred from reentering the US for a specific period of time.
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