On your child's page, recent taps are listed with an Undo button beside each one for 5 minutes. It removes those points from the total and from every prize bar, exactly as if the tap had not happened.
The 5-minute limit is deliberate. Undo is for a slipped finger, not for changing your mind about whether they deserved it an hour later — and a child who learns points can be reclaimed stops trusting all of them.
The one thing undo never walks back is the big prize once it has been reached. That one stays won.