It didn't understand use strict; properly - then you'll provide it with good reading.
Here, at the monastery, there are plenty of excellent articles about this subject.

Much worse will be if this child refuses to learn and did not respect your programming practice when you're explained it about 'strict' - but presumably this is not the case. You just tried to not get angry but anyways got angry when a person did not understood obvious for you thing.

I learned from many articles here, that good programming is about learning many things (programming techniques, behaviour in the office, corporate etiquette, and many many things).
You should give a chance to learn to the child.