For plans that have not been paid, you can not see that there are child plans involved from the subscriptions tab you have to go to the basket to see them all.
If you try to delete an unpaid plan that has a child plan from the subscription tab, you get an error about child plans still existing (don't remember the exact message)and you can't delete it.
I did (through experimentation) learn that you must delete the basket and the notifications to get rid of them .... but what is the purpose of a delete option on the subscription tab if it doesn't work.