"The parent said, 'Only through questioning teachers and school administrators did we learn the researchers would put stickers on children who opted out, but no further information was provided on whether they would still be filmed.'"
That labeling I'm sure would have social consequences- singling out people and giving them a label is a path toward exclusion in other activities and make wearing a sticker a sigma... :(
Not the star they wanted
As a practical matter, it's also just stupid. Preschoolers will take off stickers all the time.
edit: fixed piratical typo