Subscribe to children

No, sorry, there's no support for that. You need to write a custom module and user interface. You'll also have to think about what happens when child terms are added later, and how to handle unsubscribing...

I'm not sure. Actually, I'm not sure we already have a suitable hook.

What exactly do you want to do? Have the user subscribe to the parent term and automatically create subscriptions to all child terms? Recursively, i.e. grand-children? Or enter one subscription to the parent term and queue notifications for all child (and grand-child?) terms?

Either way you probably have to hook_form_alter() the taxa subscriptions form, if only to add an explanation of how this works. If you do that, you can (option #1) remove (or keep, whatever...) the child terms, and you can replace (and then ultimately call) the submit handler, and simulate the user checking the child terms.

The second option is more difficult, because this is essentially done in one DBTNG call. You may be able to hook_query_alter() (D6: hook_db_rewrite_sql()) this call to replace an '==tid' condition with an 'IN (list of tids)' condition.

There may well be other strategies besides the two I've outlined above. That's why I don't think it makes sense to try to implement this kind of functionality in Subscriptions itself.

I will consider adding hooks if that's what's needed, but we'll have to add them to D7 first (i.e. you'll have to submit patches for D7 and D6). This is in your (or at least in your client's) interest, because it ensures that there is an upgrade path to D7 and beyond. Feel free to reopen this if needed.

subscriptions/add/taxa/% goes through subscriptions_add_form(), which posts to subscriptions_add_form_submit(), which calls hook_subscriptions_stype_alter().

So, you can implement that hook and call subscriptions_write_subscription() yourself for each desired child term and then let the submit handler do its thing.