Ruby on Rails API: computed attribute pattern best practice

I would like to know the best practice when I need a computed attribute that require a call to the database.

If I have a Parent that has many Child, how would I render a children_count attribute in ParentController#index as I don't want to render the children, just the count? what's the best way to do it?

Thank you!

Model:

class Parent < ApplicationRecord
  has_many :children

  def children_count
    children.count # Wouldn't it ask the database when I call this method?
  end
end

Controller:

class ParentsController < ApplicationController
  def index
    parents = Parent.all

    render json: parents, only: %i[attr1, attr2] # How do I pass children_count?
  end
end