It's surprising that home router devices still use default passwords when the fix is so simple. Generate a random password and print it on a sticker on the underneath of the device. If anyone has physical access to the device, they must be its owner anyway. This wouldn't prevent your children overriding parental controls but it would be good enough for every other home setup.
Or, indeed, make a small switch on the front panel that must be pressed to enable administration mode. Passwords are quite unnecessary when you can just test that the user can physically touch the device.
A third alternative would be to disallow administration over the wireless network; only someone connected over an Ethernet cable can do it.