Why was my child disqualified, and who can make that call?

Under Article 102.21:

• Only the official responsible for that part of the race (Referee, Stroke Judge, Turn Judge or Relay Take-off Judge) can disqualify a swimmer. They must raise one hand overhead when they see the infraction. If they don't, there's no disqualification.
• The Referee confirms the disqualification. The official must make every reasonable effort to find the swimmer or their coach and explain why. Not being told doesn't cancel the disqualification.
• A disqualified swim gets no time or place in the results.

Questioning a call: judgment decisions by starters and stroke, turn, place and take-off judges can be reviewed only by the Referee, and the Referee's decision is final (102.22.1). Other protests must be made to the Referee in writing within 30 minutes of the race (102.22.4). The usual first step is to ask your child's coach to talk to the Referee.