I had this same issue a few days ago, and I found it.
1. Log into MY T-MOBILE.
2. Click Profile on the top right corner.
3. Scroll down to Family Controls.
4. You should see Family Where in that list.
5. Click Settings under FamilyWhere.
Were you able to use the steps above to cancel the feature? If it's not available, you should be able to cancel or not opt in to the feature after the trial is up.