It’s likely something to do with the CSS in your child theme.
You’d need to go through them and try to identify the issue that way.
The easiest way to debug problems like this is to disable all of the CSS in your child theme, and slowly add them back in to identify the problematic code.
If you need to child theme to be activated then it’s best to start a staging site so you can take some debugging steps and keep your main site running.
You could also contact your developer and ask him to sort it out for you.