I don't see the problem? As gamers we buy DLC, ingame stuff, it's just the way it is.
The way i used to deal with it is pocket money. It is their money and therefore free to spend on what they like, but once it's gone, it's gone. They soon realised that after spunking all their money V-Bucks, they had nothing left for sweets, a new game, treats etc etc.
So they learned to balance their money, ie a little on V-Bucks and kept the rest for other stuff.
Now my daughters older, she actually said to me she couldn't believe how much she spent on V-Bucks when younger, she could have bought new clothes/make up with the amount - lol.