DHCP leases are not significant/relevant here. (click for more)
DHCP has no control over DNS, it just may suggest a client OS to use a certain DNS resolver.
Independently, client software may be configured for alternate DNS resolvers in a multitude of ways, none of them requiring tampering with DHCP leases.
In addition, since some OSs started to enable MAC address randomisation for wifi access by default, without further measures you may not be able to identify your kid's devices by looking at MAC addresses alone.
Yes, having your router's firewall redirect port 53 traffic in your network does address one possibility of by-passing Pi-hole, and it makes sense to apply it - provided your router supports it.
But it does not address other kinds of by-passes which can be configured on a client, like forcing a browser's usage of DoH or a smartphone using DoT, or using a VPN.
This serves to demonstrate and underpin my previous statement:
Some parting thoughts, as this is now quickly moving further and further away from Pi-hole. ;) (click for more)
DNS blocks can play a (minor) part in parental control strategies, but they do not exactly lend themselves to time-based controls, and they won't obsolete other network-wide and on-device measures - and certainly neither non-technical, more important educational measures.
I personally believe that any such technical action you take is only a fraction as effective as face-to-face attention and direct conversation with your children.
I think you should do both - protect them while they are still too young, but also teach them how to fend for themselves as they grow older - which would mean explaining why you apply DNS filters, why it is a good idea to do so, and why they should vow they won't try to evade it.
It may make sense to agree on a solution for age-appropriate usage together with your kids, and additionally use Android's or Apple's on-board parental control tools, which can be used to implement device-related (and not only home network-related) security.
The disadvantage of this is that the necessary linking of accounts means that family relations and other individual data must also be disclosed to the respective providers.
Further discussion may be more well-suited for forums specialising in parental control. 