Your teacher is correct.
What it really boils down to, is the purpose of the checkpoint. Checkpoints fall under what is known as a special needs exception to the 4th Amendment.
DL checkpoints are legal, insurance checkpoints are legal, DUI checkpoints are legal.
A checkpoint to search a car is not. The USSC decided that when they stated that walking a drug dog around a car at a narcotics checkpoint was not legal. Indianapolis v. Edmond
If the person with the child, was stopped at a DL checkpoint for example, and the officer noticed the child in the car, that's a different story. In your scenario, they are stopping every car, without probable cause in order to search the cabin for the missing child.
Key points there are 'without probable cause' and 'search'.