How to update the default device for family sharing ask to buy approvals

I have Family Sharing Ask to Buy set up for my two kids. When they go to request an approval, the approval defaults most of the time to my iMac and not my iPhone. I have my personal Apple ID set up on my iPhone, iWatch, iPad, iMac and MacBook. I have to be at my computer at the time they enter the request in order to approve it. At one point the approval requests went to my phone then all of the sudden stopped. In some cases, the kids will enter the request and it doesn't go to either, it might even go to my iPad and I'm not aware. This just happened the other day, he had to enter the request 3 times while I was sitting at my desk and finally on the 3rd time it showed up on my iMac, never coming to my iPhone that was sitting next to me. I have verified that notifications are turned on for this and that Ask to Buy are set up correctly as the organizer and for the kids for sharing. Please advise how to set it up that the notification defaults to my iPhone and not any of the other devices. This is very inconvenient when I am working or away from my home and cannot approve their requests. Thank you!