Is there anyway to lock an iphone to avoid my toddler from leaving his video? I have a toddler iphone app that plays short videos.  He loves it and its great to give to him in the car, etc but the one issue is that he keeps hitting the home button and i have to start the app up again.  This happens over and over again.  

I noticed there is no lock button on the iphone.  Why is that?  Is there any solution to get the equivalent of what a lock button would do here so going back home is disabled?
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.