Can a Legal guardian of a child file for their green card?

Answer Yes

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If the legal guardian of a child let the child visit his mother and she does not return the child how do you get the child back?

Answer Contact the local police and request they accompany the guardian to the residence where the child is located. This is not an ideal solution, however a court appointed guardian is responsible... Read More »

Can an American citizen adopt a legal immigrant and if so would that child get citizenship and or a green card?

Yes, but the child will have to follow steps in immigration procedures and apply for naturalization later.

Can a legal guardian apply for and receive child support from the parent of the child?

Answer I don't see any reason why not. For more information see

Can a child decide who their legal guardian is?

No they can not and a minor can not decide who to live with.