form_for to dynamically add multiple child objects

I have a data structure as follows:
Task has_many Reminders

Now I would like to have users able to specify one or more reminder in
the task form at the time when user creates the task. How could I do
that? The same issue applies when I have one or more reminders being
created, I would like to allow the user to modify all these reminders
same time when they are modifying the task.

Check out the railscasts series on complex forms:

http://railscasts.com/episodes/73
http://railscasts.com/episodes/74
http://railscasts.com/episodes/75

Should be of help.