Apple has a new service called Family Sharing that really simplifies the process of managing purchases for kids. In the past it was possible — and legal — to share apps and music among family members, but it wasn't easy. What was easy was to allow children unrestricted access to the App Store. All those free games, right? There were shocks lurking in credit card statements for some people. Things have changed for the better, and I recommend setting up Family Sharing if your kids still need monitoring for a few years.
Family Sharing is also useful for saving money on apps, books, and music when there's more than one person in the household. No kids necessary.
There's a lot to it, both in features and setup, but you can read about Family Sharing here.