user_serializer

class UserSerializer < ActiveModel::Serializer
  attributes :id, :name, :phone, :email, :balance, :children, :balance 

  def attributes
    data = super
    data[:children] = User.find(data[:id]).subordinates.any?
    data
  end
  
end

users_controller

if request.format.json? && params[:id] != nil && have_permissions?(User.find(params[:id]))
  render json: @subusers, root: false
end