Hello JuanPC,
Thanks for asking the Apple Support Communities for help with creating an Apple ID for your child.
Please ensure you are following the steps in this help article: Family Sharing and Apple ID for your child - Apple Support
If you still have trouble, it's possible this may also be related to the network you are connected to. If you are using Wi-Fi, I recommend trying while connected to a different wireless network. You can also try by turning off Wi-Fi and testing with your cellular connection only.
- To turn Wi-Fi off and use your cellular connection, go to Settings > Wi-Fi, and turn the slider off. Be aware that this will use your cellular data until you turn Wi-Fi back on.
Thanks and have a wonderful day.