Quote Quoting llworking
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That would be true if the divorce decree specifically awarded the 401k to the OP. It would be a different story if the 401k was divided in the divorce and the OP was not cooperating with that because mom wasn't paying child support. The OP really needs to be clearer about what is going on.
I agree that it depends a great deal on what happened during the divorce proceedings and what, if anything, the court decided with respect to the § 401(k) account. The OP needs to provide some more information on that to get much in the way of helpful comments.

(By the way, it is not a 401k, 401K, or any of the other variations I have seen people use; it is properly written as 401(k) as it is IRC § 401(k) that provides the rules for these accounts.)