Lock Cap for Permanent Markers

thingiverse

To keep little ones from unscrewing marker caps, try using a device that screws them on tighter, like a marker cap lock. This handy tool fits onto a standard marker and secures it so securely, even the most determined tiny hands won't be able to get it open. Another option is to store markers with their caps screwed off in a safe place where they can't reach them. You could also try labeling the markers with a warning sticker to deter your kids from trying to use them.