> We face regular discussions about why he shouldn't spend $30 on Robux or some other digital currency. I try and explain that these digital currencies don't have real value like, say, a tennis racket or a pair of shoes.

I’m not sure I agree. Value is a deep and multi-faceted concept. It’s very likely that your child would value the games they can purchase with Robux(it’s my understanding that Roblox has a bunch of games that can be purchased?).