Read the following support article Request and make purchases with Ask to Buy - Apple Support it explains how this setup works. Note the following lines
"Open the notification to see the item that your family member wants to get. (See your notifications on your Mac or iOS device.)"
This is the notification centre you access by pulling down from the top of your device and then clicking [Notifications] at the top of the screen. There will be a section for "Family" that will show the request on all parent devices. (iOSv8.4). This is the only place that the requests are sent to, it doesn't go anywhere else.
"If the organizer doesn't make the purchase or decline the request, it will be deleted after 24 hours, and the child will need to make the request again."
If either parent doesn't action the request its deleted after 24 hours and the child needs to re-request it. So basically you have 24hrs to approve or reject the purchase.
Also, as mentioned in a post above, make sure your child's device is signed into iCloud and iTunes as their child account (look under [Settings] >> [iCloud] & [Settings] >> [iTunes & App Store]). You may also want to configure iMessage and Facetime to use the childs account as well.