This idea is meant to follow a program focusing on making promises to God.
Once they have chosen a specific promise that they want to make to God have them choose a symbol that represents that commitment. It could be a symbol of their own making, but you can also look up some Viking Runes, Hebrew letters, Chinese characters, etc.
Then tattoo that symbol somewhere on their body to represent their commitment to God that night.
Depending on the kids, permanent markers is the best. But if you use permanent markers, you should show some YouTube video’s about how you can remove permanent marker in case their parents have an issue with it. (there are a ton! And it’s actually easy!)
Otherwise you can use washable markers, so parents won’t be upset.
If some parents are not ok with any writing on their bodies, they can write it on a paper and make a bracelet or necklace with it.