How can I choose a taxon with their children included - Spree Ecommerce (Ruby on Rails)

I need to select affordable taxa and their children.

I use this custom rule:

module Spree
  class Promotion
    module Rules

      class TaxonPromotionRule < Spree::PromotionRule

        has_and_belongs_to_many :taxon, class_name: '::Spree::Taxon', join_table: 'spree_taxons_promotion_rules', foreign_key: 'promotion_rule_id'
        validate :only_one_promotion_per_product

        MATCH_POLICIES = %w(any all)
        preference :match_policy, :string, default: MATCH_POLICIES.first

        # scope/association that is used to test eligibility
        def eligible_taxons
          taxon
        end

        def applicable?(promotable)
          promotable.is_a?(Spree::Order)
        end

        def eligible?(order, options = {})
          return false if eligible_taxons.empty?
          if preferred_match_policy == 'all'
            eligible_taxons.all? {|p| order.products.include_taxon?(p) }
          else
            order.products.any? {|p| eligible_taxons.any? {|t| t.include_product?(p)} }
          end
        end

        def taxon_ids_string
          taxon_ids.join(',')
        end

        def taxon_ids_string=(s)
          self.taxon_ids = s.to_s.split(',').map(&:strip)
        end

        private

          def only_one_promotion_per_product
            if Spree::Promotion::Rules::TaxonPromotionRule.all.map(&:taxon).flatten.uniq!
              errors[:base] << "You can't create two promotions for the same product"
            end
          end

      end

    end
  end
end

and decorator:

Spree::Taxon.class_eval do
  def include_product? p
    products.include? p
  end
end

I want eligible_taxons to be a taxon from the rules table and the entire child identifier. Therefore, if I set some root category, this rule will apply to all child categories. Hope my question is clear and clear. :)