Neither of you are being very convincing HTML5 games are safe. I'm trying to understand this. So you can't trick a child into downloading an executable file from within a game, maybe telling them it's an add-on that allows for more features, and then have a button that executes that downloaded file as described in the linked thread?
...but as long as Windows is kept up-to-date, you're often safe.
A child shouldn't be admin on a computer...
There are better things to worry about regarding children online.
You're both making a lot of assumptions about how responsible parents are or what a child should or shouldn't have access to on personal computers and what parents should be worried about online. We can debate all of that, but I'm not really interested in those opinions. I'm just curious to know if HTML5 games are safe.