Why can't I approve my child's request using TouchID?

I've given my old 5S to my 3 year old to use for apps and movies. I turned on "Ask To Buy" on his phone, since that seemed like the thing to do. (I wish turning off the app store under restrictions would stop paid kids' apps from relentlessly presenting ads for other apps, but that's another discussion.) Now, when I want to put an app on his phone, I have to:

Go into the app store, possibly enabling it under restrictions, first

Enter the password for his AppleID

Enter the password for his AppleID, again.

Acknowledge the Ask To Buy request should be sent.

Switch to my phone

Open the Ask To Buy notification

Enter my AppleID's password (Touch ID is apparently not supported, although I can buy apps for myself with TouchID)

Switch back to his phone

Open the Ask To Buy approved notification

Enter his password, AGAIN

Download the app.

How did this seem like a good workflow to anyone in Cupertino?