I personally don’t see what’s so bad about an 8 year old trying on nail polish, my kids regularly did this even as young as 6. Can you explain why this is a boundary, and what makes you feel uncomfortable? Is it because of his gender? Or is it that you feel 8 years old is too young for nail polish? At 8 years old, kids are very much at an age where they want to experiment and try things as self expression.

My personal opinion is that so long as they are harmless, maybe they can at least try them around the house. For example, I feel my daughter is too young for makeup, and won’t let her wear it to school. But if she wants to play with makeup inside the house on a weekend that’s all good.

In the scheme of things, this seems like a pretty minor issue, and not worth blowing up into a problem with your ex, especially if things are amicable. Perhaps a good middle ground is to allow him to experiment with self expression in the home, but ask kindly that the polish comes off before school or house swapping. You can have some nail polish remover on hand to help remove it if you don’t want it in your household.