diff --git a/app/models/derived_variables/lettings_log_variables.rb b/app/models/derived_variables/lettings_log_variables.rb
index fc748460a..9ea820dc0 100644
--- a/app/models/derived_variables/lettings_log_variables.rb
+++ b/app/models/derived_variables/lettings_log_variables.rb
@@ -246,8 +246,13 @@ private
   end
 
   def get_totchild
-    relationships = [relat2, relat3, relat4, relat5, relat6, relat7, relat8]
-    relationships.count("C")
+    if form.start_year_2025_or_later?
+      ages = [age1, age2, age3, age4, age5, age6, age7, age8]
+      ages.count { |x| !x.nil? && x < 16 }
+    else
+      relationships = [relat2, relat3, relat4, relat5, relat6, relat7, relat8]
+      relationships.count("C")
+    end
   end
 
   def get_totadult
diff --git a/app/services/feature_toggle.rb b/app/services/feature_toggle.rb
index 4b2c440fc..827e87da9 100644
--- a/app/services/feature_toggle.rb
+++ b/app/services/feature_toggle.rb
@@ -1,6 +1,6 @@
 class FeatureToggle
   def self.allow_future_form_use?
-    Rails.env.development? || Rails.env.review? || Rails.env.staging?
+    Rails.env.development? || Rails.env.review? || Rails.env.staging? || Rails.env.test?
   end
 
   def self.bulk_upload_duplicate_log_check_enabled?
diff --git a/spec/models/lettings_log_derived_fields_spec.rb b/spec/models/lettings_log_derived_fields_spec.rb
index c7444a956..a6e19f5c4 100644
--- a/spec/models/lettings_log_derived_fields_spec.rb
+++ b/spec/models/lettings_log_derived_fields_spec.rb
@@ -2,9 +2,12 @@ require "rails_helper"
 require "shared/shared_examples_for_derived_fields"
 
 RSpec.describe LettingsLog, type: :model do
+  include CollectionTimeHelper
+
   let(:organisation) { build(:organisation, name: "derived fields org") }
   let(:user) { build(:user, organisation:) }
-  let(:log) { build(:lettings_log, :startdate_today, assigned_to: user) }
+  let(:startdate) { current_collection_start_date }
+  let(:log) { build(:lettings_log, startdate:, assigned_to: user) }
 
   include_examples "shared examples for derived fields", :lettings_log
 
@@ -111,10 +114,10 @@ RSpec.describe LettingsLog, type: :model do
     before do
       log.assign_attributes(
         relat2: "X",
-        relat3: "C",
+        relat3: "X",
         relat4: "X",
-        relat5: "C",
-        relat7: "C",
+        relat5: "X",
+        # relat7 is derived
         relat8: "X",
         age1: 22,
         age2: 16,
@@ -128,7 +131,7 @@ RSpec.describe LettingsLog, type: :model do
     end
 
     it "correctly derives totchild" do
-      expect(log.totchild).to eq 3
+      expect(log.totchild).to eq 1
     end
 
     it "correctly derives totelder" do
@@ -142,6 +145,22 @@ RSpec.describe LettingsLog, type: :model do
     it "correctly derives economic status for tenants under 16" do
       expect(log.ecstat7).to eq 9
     end
+
+    context "when it is 2025", metadata: { year: 25 } do
+      let(:startdate) { collection_start_date_for_year(2025) }
+
+      it "does not derive relationship for tenants under 16" do
+        expect(log.relat7).to be_nil
+      end
+    end
+
+    context "when it is 2026", metadata: { year: 26 } do
+      let(:startdate) { collection_start_date_for_year(2026) }
+
+      it "derives relationship for tenants under 16" do
+        expect(log.relat7).to eq "X"
+      end
+    end
   end
 
   describe "deriving lettype" do
@@ -1213,23 +1232,87 @@ RSpec.describe LettingsLog, type: :model do
     end
   end
 
-  describe "#clear_child_ecstat_for_age_changes!" do
-    it "clears the working situation of a person that was previously a child under 16" do
-      log = create(:lettings_log, :completed, age2: 13)
-      log.age2 = 17
-      expect { log.set_derived_fields! }.to change(log, :ecstat2).from(9).to(nil)
+  describe "#clear_child_constraints_for_age_changes!" do
+    let(:startdate) { current_collection_start_date }
+    let(:log) { create(:lettings_log, :completed, startdate:, age2: initial_age2) }
+
+    before do
+      log.age2 = updated_age2
     end
 
-    it "does not clear the working situation of a person that had an age change but is still a child under 16" do
-      log = create(:lettings_log, :completed, age2: 13)
-      log.age2 = 15
-      expect { log.set_derived_fields! }.to not_change(log, :ecstat2)
+    context "when person was previously a child under 16" do
+      let(:initial_age2) { 13 }
+      let(:updated_age2) { 16 }
+
+      it "clears the working situation" do
+        expect { log.set_derived_fields! }.to change(log, :ecstat2).from(9).to(nil)
+      end
+
+      context "and it is 2025", metadata: { year: 25 } do
+        let(:startdate) { collection_start_date_for_year(2025) }
+
+        it "does not clear the relationship" do
+          expect { log.set_derived_fields! }.to not_change(log, :relat2)
+        end
+      end
+
+      context "and it is 2026", metadata: { year: 26 } do
+        let(:startdate) { collection_start_date_for_year(2026) }
+
+        it "clears the relationship" do
+          expect { log.set_derived_fields! }.to change(log, :relat2).from("X").to(nil)
+        end
+      end
     end
 
-    it "does not clear the working situation of a person that had an age change but is still an adult" do
-      log = create(:lettings_log, :completed, age2: 45)
-      log.age2 = 46
-      expect { log.set_derived_fields! }.to not_change(log, :ecstat2)
+    context "when person had an age change but is still a child under 16" do
+      let(:initial_age2) { 13 }
+      let(:updated_age2) { 15 }
+
+      it "does not clear the working situation" do
+        expect { log.set_derived_fields! }.to not_change(log, :ecstat2)
+      end
+
+      context "and it is 2025", metadata: { year: 25 } do
+        let(:startdate) { collection_start_date_for_year(2025) }
+
+        it "does not clear the relationship" do
+          expect { log.set_derived_fields! }.to not_change(log, :relat2)
+        end
+      end
+
+      context "and it is 2026", metadata: { year: 26 } do
+        let(:startdate) { collection_start_date_for_year(2026) }
+
+        it "does not clear the relationship" do
+          expect { log.set_derived_fields! }.to not_change(log, :relat2)
+        end
+      end
+    end
+
+    context "when person had an age change but is still an adult" do
+      let(:initial_age2) { 45 }
+      let(:updated_age2) { 46 }
+
+      it "does not clear the working situation" do
+        expect { log.set_derived_fields! }.to not_change(log, :ecstat2)
+      end
+
+      context "and it is 2025", metadata: { year: 25 } do
+        let(:startdate) { collection_start_date_for_year(2025) }
+
+        it "does not clear the relationship" do
+          expect { log.set_derived_fields! }.to not_change(log, :relat2)
+        end
+      end
+
+      context "and it is 2026", metadata: { year: 26 } do
+        let(:startdate) { collection_start_date_for_year(2026) }
+
+        it "does not clear the relationship" do
+          expect { log.set_derived_fields! }.to not_change(log, :relat2)
+        end
+      end
     end
   end