My child is sick or injured on race day. Can the crew substitute another rower?

Usually yes, within limits (Rules of Rowing 5-206 and 5-207):

• Normal substitutions: after entries close, a crew other than a single scull can replace up to half of its rowers plus the coxswain. The regatta can refuse a substitution that isn't submitted in writing at least one hour before the crew's first race in that event.
• Once racing starts: after a crew's first race in an event (for example, a heat), the lineup stays the same for the rest of that event.
• Sudden serious illness or injury: if the organizing committee agrees, that rower can be replaced at any time before the event is finished, and it doesn't count toward the half-crew limit. A certificate from a health professional is normally required.
• Single sculls (1x): a sculler can be replaced only for serious, sudden illness, and only before the first race of the event.
• Returning to race: a rower replaced for illness or injury can't race again at that regatta until restored to good health, normally with a health professional's certificate.

Any substitute must be eligible for the event in their own right, for example as a Youth rower or within the lightweight limit.