If you are going to take on a junior then as far as I am concerned you are duty bound to be that person's point of contact and mentor.

The most important thing for a junior is NOT to be made to feel like their an inconvenience for asking questions.  It is also important to introduce them to key subject matter experts who also regard mentoring juniors as a holy duty.  Time invested in juniors pays dividends.  They get to be productive very quickly and they learn the care and share behaviour from you.  Corporate behaviour flows down hill.

Teach once learn twice