Enabling it on one should make it work across all your Apple products provided the products are all signed into iCloud using the same username.
I’d suggest going into iPhone settings->your name -> family sharing and see if iPhone is showing ‘Ask to Buy’ enabled for each of your childrens’ name in the list (they should show enabled if you did that on your Mac but it could be a quirk that it needs to be set on iPhone first??).
If you found Ask To Buy was enabled, try going into settings->notifications. Allow notifications on?
Never hurts to try a reboot ??????