I was about to write "I wish I could be a child again" but Grammarly said "I wish I was a child" was correct.
Both are correct English. They do not mean exactly the same thing.
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Both of yours are correct, and "I wish I were a child again" is also correct.
CJ
isthisuniqueenough Grammarly said
Grammarly didn't say anything. Grammarly is a computer program. Computers can't do grammar or even spell. They can't love you back. They can't drive cars. They cannot converse or compose. They will remain primitive machines for the foreseeable future, with about one trillionth of the intelligence of the dumbest student.