I cant play online with my child account

As mentioned only the people of age and in charge of the family account can makes the changes for you. All they need to do is this basically

Go to account.xbox.com website. Log in under parent account.

Go to "privacy & online safety" page.

on the page you should see your profile and your child(s).

click on the child profile pic, it should be green now.

click the "Xbox One Online Safety" tab.

Allow everything but "buy and download".

Click save at the bottom!

click the "Xbox 360 Online Safety" tab.

allow the first 2 "Accept Friend Requests" and "Online Gameplay".

make sure you click save.

I'm assume your childs account is member of the Family Accounts.

Xbox One App: How to Allow Access Under Family Accounts

1. To begin, log into the main account that has control over the Family accounts.
2. After that, go to All Settings -> Account -> Family.
3. Once there, choose the child account for which you wish to allow access.
4. Next, go to Privacy & Online Safety -> Xbox Live privacy -> View Details & Customize.

Hey there,

Have you given the child account permission by signing in with your adult account?

This article, from Xbox Support may be of some help, hopefully!

As others have mentioned, you need to check your settings and that of the child account to be sure that online play is enabled. A lot of things like this are disabled by default so you need to go in and manually change them.

Youll have to ask your father can fix it on his adult account