My nieces did that to my mom's medication case. Everyone was happy but then my mom looked somewhat uncomfy and she confided to me later that day "I'm glad they did that but these stickers are ugly... But it's so so nice!!!!" She really needed to get that off her chest and still uses the same decorated case years later^^
I Love this! .. but there's a risk that the stickers adhesive degrades the plastic, making the hardhat less effective. Maybe this should be an ornament instead?
(I work for a hardhat manufacturer)