Is there any way to show scrollbar in child div when I don't know the length of parent div

Question

I have a parent div, as it's a sub section of whole complicated page, I can't get it's actual height.
Is there a way to show a vertical scrollbar in child div?

<div id="parent" style="display: flex; flex-direction: column;">
  <div id="title">This is title</div>
  <div id="children" style="flex: 1 1 auto; overflow-y: auto;">
     thousand lines...
  </di>
</div>

Answer 1

Use overflow-y: scroll

mdn reference on overflow-y