If your child set a passcode to the screen lock and cannot tell you what he/she set, restore the iPod from the computer to which the iPod is synced. For information and instructions, see:
If that will not work, you'll need to put the iPod into Recovery Mode and then try the Restore again:
If that still doesn't work, as a last resort try DFU mode:
Forum Tip: Since you're new here, you've probably not discovered the Search feature available on every Communities page, but next time, it might save you time (and everyone else from having to answer the same question multiple times) if you search a couple of ways for a topic, both in the relevant forums and in the Apple Knowledge Base, before you post a question.