There is evidently some confusion in regards to the "Ask to Buy" feature. You are expecting it to do something outside of it's intended purpose. I am not saying there is not prudence in also developing a feature to do what you want, but that is not the intended purpose of the feature. "Ask to Buy" is simply a parental control which parents can decide to enable or not. It's sole purpose is to allow parents to approve the content... It has nothing to do with payment.

DDwx in an earlier reply clearly outlines how the billing works.

No, ask to buy is about finance. Parental controls are about controlling content.

From the Apple Website ...

Request and make purchases with Ask to Buy

With Ask to Buy, you can give kids the freedom to make their own choices while still controlling your spending.

There are quite a few posters who don't like that "Ask to buy" occurs even when the child account has credit enough to cover it.  Some have even asked why?  For myself, I am very happy that I have to give approval over any / all purchase requests by my kids.  Even if the app is free, I still need to approve it, and I think that is a fantastic feature.  Why?  Because there are apps / things out there that I don't want my kids to have on their phones even if they are free.  Games that are not age appropriate; internet apps that can be abused; chat apps that have reputations for inappropriate messaging / pics; songs / videos that are adult rated, etc.  I want to protect my kids from some of the stuff out there, and if they didn't have to ask permission before getting a new app, then that defeats the purpose of activating the "Ask to buy" option.

If you trust your kids enough to not want to worry about approving app purchases when they have the credits for them on their accounts, then just turn off "Ask to buy" and tell them that if they want to buy anything that costs more than they have credit for, they need to talk to you first.

Maybe Apple will implement some customization options where you can have "Ask to buy" but only for non-free things.  

Maybe Apple will implement some customization options where you can have "Ask to buy" but only for non-free things.  

That would be the obvious and best solution. However, until then, the built in restrictions cover what you would like to do perfectly. Therefore, no need to duplicate functionality?

Note the wording in the concept from Apple. Doesn't match the current implementation.