b) permit me to put a code on a given category.
I do not know of a front end that has this feature. I could possibly implement something in the future to FrontEndHelper where an xml file with unofficial ratings is read, and mame is immediately paused if a restricted game launched, requesting authentication to continue or exit. However, that is not something that I could make happen anytime soon.
For now, I would either create a partition (or separate drive) and duplicate your install. One install have all of your games, and the other limited. Then hide an arcade button inside the cabinet accessible by locked coin door to enter bios on boot, so you can select which partition or drive you want to boot. You could possibly crete a script to change boot drive back to restricted installation, launched at shutdown of frontend.
All this could be avoided though if you watched your kids and built up trust with them. It's probably the easiest solution.