Apple's iOS 8 introduces Family Sharing, a feature that allows multiple members of the same family to share app purchases with one another, solving the long-standing problem where multiple family members had to share a single iTunes Store account. Now, Family Sharing allows each person to have their own, individual Store accounts, all linked together with purchases shared among them.
However, Family Sharing goes further than the Store. In addition to getting immediate access to each other's music, movies, TV shows, books and, yes, apps, you also are added to a family photo album as well as a family calendar. Additionally, location tracking is set up to more easily see where the family is and also to find lost devices.
It's these extra features that have confused many users, however, because all of these are tied to your main iCloud account. That works out fine if all your purchases are also tied to that same account, but for many families who set up a separate iTunes Store-only account that's not the case.
The good news is that Apple thought of this and has built-in functionality to address it. The bad news is that the setup interface doesn't make this entirely clear until you're deep into the process.
Change To The Account With All Your Apps
The first thing you need to do is set up Family Sharing on your main iCloud account. Given that your main goal for this will be to share purchases that may seem counter-intuitive, but trust us... it's how it works.
Once you've got it setup, you then go into Settings > iCloud > Family > (Me) and tap on the Apple ID account listed under "Family Purchases." When you do this your device will warn you that changing this account will alter which purchases your family has access to, but that's the goal. Go ahead and agree and change that to the account through which you purchases all your apps.
That's it. Family Sharing will remain attached to your main iCloud account, but your linked purchases will come from this separate account.
It's also possible that this will happen automatically as you set up Family Sharing (especially if your separate purchases account is listed under the Apple ID for Settings > iTunes & App Store), though it's not obvious at the start that it will.
Hopefully this helps!