Look, I hate to break it to you, but 40%+ of parents would measure dosages wrong regardless of the measurement system being used.

That's because they're morons. There's no IQ test required to breed, and that's a cause of a lot of problems in the world, never mind the US.

This issue won't be cured by changing to the metric system. 

Want to see the problem for yourself?

Go down to your DMV office. Sit there for, oh, a couple of hours. Notice the level of intellect you see in any large gathering of people.

Now, I want you to remember than half of the population is dumber than the average you just observed.