UX associate table to another

Question

It's not really about having issues with a specific language. I'm just wondering how I should display associated tables.

Here is my problem. I have to build an app where, as a parent, you can register your children to different activities. For example, let's say you want to register your child 'Henry' to 'Football' Activity. My first idea was to make a dropdown button for each Child where it displays his Activities after clicking. But how do the child register to an activity ? How can I display that ?

Besides, we could also put a dropdown button in Activities Table where we see the registered children.

To better understand, I have a database with 3 tables : Child, Activity and Subscription which allows me to make the association between Children and Activities. (Each child has a parent and in this case I display the parent's children)

Example of records : (I want to associate Henry to Football and Museum)

Image (I can't upload images yet since I'm new)

I hope I was clear enough...

I'm totally loss and I don't know what I should do. I would appreciate some insight. Thanks in advance! :)