From the admin site, when looking at a subscription tab for someone who chose pay off-line there is a "Pay Now" check-box. To me the natural thing for this to do would be the equivalent of going to the Baskets page and marking the basket paid for that user; which processes all subscriptions (Parent, Child and Donations) as well as marks the basket complete. It does not do this.
What it does is process something that is part of a basket all by it's self. Items such as this only show the parent plan, the child plan(s) are not visible (at this time) on the subscription tab for unpaid items. If you select the box and save, only the parent item is marked paid (even though it has a mandatory child plan). Open the profile again, now you can see the child. Select it, save again. Even after all subscriptions are marked paid the basket still says "pending" (open baskets left hanging around).
Being able to process half a basket to me seems like a very questionable "feature", even when only the Admin can do it. I would consider it far better to have to cancel/delete the basket (see my other post on that) and then separately process the "new" request.