I have signs like this on all the doors and gates that lead to the outside. I printed the sign out in several sizes and laminated the big ones. I also printed out a few stickers (using sticker paper in my home printer) and stuck them right to the door knobs.
My kiddos view the sign (shape, size, color and the visual) kind of like those old school "Mr Yuck" stickers we used to get...
When I started using the signs/sticker, I sent a memo home with parents explaining that adults ONLY could touch, open or close the gates and doors leading outside the house and outside the property. I explained that it was for safety reasons and that I FULLY expect ALL parents to adhere to this rule as well as teach their child.
I also laid out consequences for not enforcing the rule. (hand to hand pass over of child, immediate departure from property) and if necessary termination from care.
I will NOT allow a parents lack of attention to safety rules ruin everything I've worked for.