My Touch ID icon is no longer visible in system preferences. How do I reset Touch ID?

I have no idea what your child did when playing with the computer. The only solution I can think of is they may have hidden the system preference. Please try:

How to Hide Preference Pane Icons in Mac OS X

1. Open System Preferences using your favorite method (Apple menu, Spotlight, Dock, etc).
2. From the menu bar, go to the “View” menu and click on “Customize…”
3. Now, you should see checkmarks next to each Preference Pane icon; simply uncheck the ones that you want to be hidden.
4. Click on “Done” at the top left corner of...

If that does not solve the problem then I would recommend you call AppleCare. If you are in the USA that number is (800) 273-2275 and outside the USA please use AppleCare contact Information to find the number for your area.

Please restart in Safe Mode and then restart normally.

Use Safe Mode to isolate issues with your Mac

Very good, glad you solved it!