If I owe back child support and I wanted to cash out my 401k, will I be able to get it?

All other things being equal, you should e able to get it. I'm sure you know there is a tx penalty you'd have to face. IF, however, there is a judgment against you for the back child support, an interception of the proceeds is far more likely. You might want to call the people who administer your 401(k) and ask them if such an interception or attachment can happen under their rules and policies.