Until they reset the BIOS password with that magic jumper on the motherboard or pull out the battery....
Yes, but then they've left evidence of their activities. That's almost as good as preventing it from a certain angle.
That is until the child realizes that they could just enable a BIOS password of their own creation. Then when the BIOS password you set eight months ago doesn't work the child plants a seed of self doubt, a simple suggestion that old dad has forgotten yet another thing.