Tapping a parent actually signs a child in (sorta)

- when a parent is tapped on the signin screen, a record is created in
the child_events table with start_time set to Time.now, and parent_id
set to the tapped parent
- we need to figure out a careful system for determining if a child is
currently signed in or not, and for determining which event to update
the "actual start time/end time" field for. The practice of storing a
child's schedule and their actual arrival/departure times may not be the
best way to do this (will discuss)