How do I see the child relationship name?

Check from Setup> Object Manager.

1. Go to the Object Manager and check the list of fields of the corresponding child object.
2. Click on the field name of the primary object's reference field in the child object's field list.
3. The content displayed in the child relationship name in the field's setting information is the child relationship name. At this time, if the parent object's reference field is a custom field, the child relationship name will be the one with "__r" appended to the end of the displayed child relationship name. For package objects, prefix with namespace prefix + "__".

Reference

How to find the merge field and childrelation names