My kids do this all the time. They also use the touchscreen in conventional laptop configuration (not folded-flat tablet mode) on their Chromebooks all the time. It's bizarre to me. I'm always trying to get them to use the keyboard, but they don't care. Example: enter password on the keyboard, then tap the log in button on the screen with their finger, rather than just pressing enter. ¯\_(ツ)_/¯

My 3 year-old never had access to tablets or phones but sees us using phones all the time. So when he gets curious and touches the laptop screen (something he does with our phones occasionally as well), he's shocked to see it doesn't react.