Craft 3 query children of sub categories
Building on Robin's comment, the ancestorOf parameter expects either an element ID or the actual element as a parameter.

For example, if you know the ID of the fruit category is 3, you'd use .anscestorOf(3), or if you already had the fruit category element, you'd do like so:

{% set fruitCategory = craft.categories.id(3).one() %}

{% set categories = craft.categories()
    .group('food')
    .ancestorOf(fruitCategory)
    .all() %}