IF Statement to include category, its children and post within each?

Hi,

Is there an IF statement I can use to show a menu on a specific category, its children and all post within those categories?

I have tried is_category().

I have also tried in_category() which just shows the content within the IF statement on all pages.

I can use the OR command || but this would mean i have to update the template every time i make a new post.

Thanks