How to set the child div's height to be parent's

You can change your #insideDiv's max-height CSS property from 100% to inherit. So this rule will be like this:

max-height: inherit;

You also might want to add box-sizing:border-box; if you go this route, as that will allow any borders or padding on #insideDiv to behave as (probably) desired.

references:

https://stackoverflow.com/questions/36609079/prevent-child-div-from-expanding-outside-of-parent