responsive menu button doesn't work with child theme

After succes full creating and styling wit a child them, I find out that the toggle button (responsive menu) no longer works. Don’t know from which moment.

I checked by activating the parent. This is still working.
I removed some files from the child in order to exclude them:
– style.css
– nav.php
– functions.php

I compared the source and saw no scrip. So I put this in the functions.php
Don’t kow what’s wrong.