@kris_a 

TP-Link does not provide solution for Deco to handle randomized MAC addresses. This had been brought on this forum to TP-Link Support attention some time ago. TP-Link just released new firmware 1.6 for M5, that firmware still does not address the issue of devices with randomized MAC addresses bypassing parental controls. It is not clear if even next firmware will have that feature in its roadmap.

To be fair, other networking hardware manufacturers initially brushed off randomized MAC addresses, too. I remember Cisco FAQ saying "on client device turn off MAC address randomization." Which obviously is not a viable solution. You might hear same recommendation from TP-Link Support, as they can't offer you anything else.

If you want working parental controls right now, you will need to look for a different brand of WiFi mesh. Before switching to a different brand, check with manufacturer or support forums how that brand deals with randomized MAC addresses.

Alternatively, if you don't want to lose your investment in Deco hardware, you may wish to consider buying modern router with parental controls, that does manage properly randomized MAC addresses, and connect your Deco mesh to it in Access Point mode. Router will manage home network, Deco will only provide WiFi coverage.