Why do parents feel the need to control what their children do online? I had unrestricted internet access as a child and turned out fine. Although I must say, a DNS based approach is more benign than some of the horrendously invasive alternatives.
Control or monitor? I'm pretty sure I don't want my kids around 4chan and/or kiwifarm till they're much older .....
Zero parenting experience here, but making something the "forbidden fruit" is probably how kids learn to change the DNS on their device.
As far as I can remember, the only real-world consequence was someone applying Bengay where it should not be applied.