DNS-based blocking more complete for your whole network, independent of the device settings for tech-avers users/kids. DNS-based blocking is less flexible for all users in the network - especially when you need to make exceptions for certain sites. They are also limited to your home network, unless you have a VPN server.
Therefore, for mobile devices app-based blocking is the main way to go.
Consequently, both make sense and your use case is relevant.
This is a great book based on generational data from the US across several decades. It shows that average delays have increased since the rise of smartphones.
So yes, bad parenting etc. might be a factor, but it cannot be neglected that more screen time leads to more problems, especially for non-adults.
OK, I was thinking of piHole (+ unbound) as local DNS blocker. Sure, there are other ways. Thanks for clarifying that!