Hello MatOl84,
Thank you for reaching out in Apple Support Communities. There are steps available that can help. We recommend for you to follow these steps as it may help:
Things to try first
1. Make sure that both you and your child have updated to the latest iOS or iPadOS on your iPhone or iPad, and the latest macOS on your Mac.
2. If you turned on Focus on your device, either turn it off so that you can receive notifications, or check your Focus settings to make sure that you can receive Time Sensitive notifications like Ask to Buy requests.
3. Try changing the name of your device, and check that you and your child's devices have different names. To see and change the name of a device, go to Settings > General > About > Name.
Following these steps can help and is outlined in this article: If Ask to Buy isn't working - Apple Support
If that doesn't help, please follow each of the sections within that same article as there's additional steps that can potentially resolve this issue.
Kind regards.