diff --git a/src/action/updatecategoryblockedtimes.ts b/src/action/updatecategoryblockedtimes.ts
index c29c7e4..bd351e5 100644
--- a/src/action/updatecategoryblockedtimes.ts
+++ b/src/action/updatecategoryblockedtimes.ts
@@ -1,6 +1,6 @@
 /*
  * server component for the TimeLimit App
- * Copyright (C) 2019 Jonas Lochmann
+ * Copyright (C) 2019 - 2020 Jonas Lochmann
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
@@ -19,6 +19,8 @@ import { validateBitmask } from '../util/bitmask'
 import { assertIdWithinFamily } from '../util/token'
 import { ParentAction } from './basetypes'
 
+export const blockedTimesBitmaskLength = 60 * 24 * 7 /* number of minutes per week */
+
 export class UpdateCategoryBlockedTimesAction extends ParentAction {
   readonly categoryId: string
   readonly blockedTimes: string
@@ -30,7 +32,7 @@ export class UpdateCategoryBlockedTimesAction extends ParentAction {
     super()
 
     assertIdWithinFamily(categoryId)
-    validateBitmask(blockedTimes, 60 * 24 * 7 /* number of minutes per week */)
+    validateBitmask(blockedTimes, blockedTimesBitmaskLength)
 
     this.categoryId = categoryId
     this.blockedTimes = blockedTimes
diff --git a/src/function/sync/apply-actions/dispatch-parent-action/index.ts b/src/function/sync/apply-actions/dispatch-parent-action/index.ts
index 1e1f0fc..058c685 100644
--- a/src/function/sync/apply-actions/dispatch-parent-action/index.ts
+++ b/src/function/sync/apply-actions/dispatch-parent-action/index.ts
@@ -125,6 +125,8 @@ export const dispatchParentAction = async ({ action, cache, parentUserId, source
     return dispatchSetParentCategory({ action, cache, fromChildSelfLimitAddChildUserId })
   } else if (action instanceof UpdateCategoryTemporarilyBlockedAction) {
     return dispatchUpdateCategoryTemporarilyBlocked({ action, cache, fromChildSelfLimitAddChildUserId })
+  } else if (action instanceof UpdateCategoryBlockedTimesAction) {
+    return dispatchUpdateCategoryBlockedTimes({ action, cache, fromChildSelfLimitAddChildUserId })
   }
 
   if (fromChildSelfLimitAddChildUserId === null) {
@@ -164,8 +166,6 @@ export const dispatchParentAction = async ({ action, cache, parentUserId, source
       return dispatchSetUserTimezone({ action, cache })
     } else if (action instanceof UpdateCategoryBatteryLimitAction) {
       return dispatchUpdateCategoryBatteryLimit({ action, cache })
-    } else if (action instanceof UpdateCategoryBlockedTimesAction) {
-      return dispatchUpdateCategoryBlockedTimes({ action, cache })
     } else if (action instanceof UpdateCategorySortingAction) {
       return dispatchUpdateCategorySorting({ action, cache })
     } else if (action instanceof IncrementCategoryExtraTimeAction) {
diff --git a/src/function/sync/apply-actions/dispatch-parent-action/updatecategoryblockedtimes.ts b/src/function/sync/apply-actions/dispatch-parent-action/updatecategoryblockedtimes.ts
index cc8bad4..4fda2bf 100644
--- a/src/function/sync/apply-actions/dispatch-parent-action/updatecategoryblockedtimes.ts
+++ b/src/function/sync/apply-actions/dispatch-parent-action/updatecategoryblockedtimes.ts
@@ -1,6 +1,6 @@
 /*
  * server component for the TimeLimit App
- * Copyright (C) 2019 Jonas Lochmann
+ * Copyright (C) 2019 - 2020 Jonas Lochmann
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
@@ -15,14 +15,49 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
-import { UpdateCategoryBlockedTimesAction } from '../../../../action'
+import { blockedTimesBitmaskLength, UpdateCategoryBlockedTimesAction } from '../../../../action/updatecategoryblockedtimes'
+import { validateAndParseBitmask } from '../../../../util/bitmask'
 import { Cache } from '../cache'
 
-export async function dispatchUpdateCategoryBlockedTimes ({ action, cache }: {
+export async function dispatchUpdateCategoryBlockedTimes ({ action, cache, fromChildSelfLimitAddChildUserId }: {
   action: UpdateCategoryBlockedTimesAction
   cache: Cache
+  fromChildSelfLimitAddChildUserId: string | null
 }) {
-  const [affectedRows] = await cache.database.category.update({
+  const categoryEntryUnsafe = await cache.database.category.findOne({
+    where: {
+      familyId: cache.familyId,
+      categoryId: action.categoryId
+    },
+    transaction: cache.transaction,
+    attributes: ['childId', 'blockedMinutesInWeek']
+  })
+
+  if (!categoryEntryUnsafe) {
+    throw new Error('can not update blocked time areas for a category which does not exist')
+  }
+
+  const categoryEntry = {
+    childId: categoryEntryUnsafe.childId,
+    blockedMinutesInWeek: categoryEntryUnsafe.blockedMinutesInWeek
+  }
+
+  if (fromChildSelfLimitAddChildUserId !== null) {
+    if (categoryEntry.childId !== fromChildSelfLimitAddChildUserId) {
+      throw new Error('can not update blocked time areas for other child users')
+    }
+
+    const oldBlocked = validateAndParseBitmask(categoryEntry.blockedMinutesInWeek, blockedTimesBitmaskLength)
+    const newBlocked = validateAndParseBitmask(action.blockedTimes, blockedTimesBitmaskLength)
+
+    oldBlocked.forEach((value, index) => {
+      if (value && !newBlocked[index]) {
+        throw new Error('new blocked time areas are smaller')
+      }
+    })
+  }
+
+  await cache.database.category.update({
     blockedMinutesInWeek: action.blockedTimes
   }, {
     where: {
@@ -32,10 +67,6 @@ export async function dispatchUpdateCategoryBlockedTimes ({ action, cache }: {
     transaction: cache.transaction
   })
 
-  if (affectedRows === 0) {
-    throw new Error('invalid category id provided')
-  }
-
   cache.categoriesWithModifiedBaseData.push(action.categoryId)
   cache.areChangesImportant = true
 }
diff --git a/src/util/bitmask.ts b/src/util/bitmask.ts
index a774848..2d3b529 100644
--- a/src/util/bitmask.ts
+++ b/src/util/bitmask.ts
@@ -1,6 +1,6 @@
 /*
  * server component for the TimeLimit App
- * Copyright (C) 2019 Jonas Lochmann
+ * Copyright (C) 2019 - 2020 Jonas Lochmann
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as