Is there anyway to lock an iphone to avoid my toddler from leaving his video?
You can use Guided Access to restrict access to a single app, and even ignore touch on certain (or all) parts of the screen.

To enable, first go to Settings > General > Accessibility > Guided Access. Then once you're in the app, just triple tap the home button.

See iOS: About Guided Access for more information or check out this short video.