Is someone suseptible to becoming infected with swine flu from a child who has had the swine flu mist?

Unlikely. If exposed to the living weakened virus of a child who receiver the nasal mist, you might get a free vaccine experience. You would not get the full blown disease.
No. Flumist has a "cold-adapted" version of the virus - it cannot live at body temperature, multiplies only in the nose, and does not cause a deeper infection.