S.B.
When I was about 3 I ate nearly a half a bottle of chewable baby tylenol. (I remember the entire event vividly!) My mother had been up with my <6 month old sister all night who was sick so I was trying to get my own breakfast. (sounds lame, but it's what goes through a 3 yr old's mind). The only reason I lived is I woke my mother up within a half hour, which I only know because the cartoon I was watching hadn't ended. I wanted to brag that I was a big girl and got my own breakfast. Okay, that was lame too, but again, I was 3..... I knew enough to know it was medicine, but not enough to know it could hurt me. Incidentally, my mother kept the bottle in a cabinet above the stove. She thought it was inaccessible because of the hood over the stove. However, like most children I was able to get to it quite easily. As a result of this incident I keep most of my medication in a location that is physically impossible for her to reach, even if she got a chair or step stool. Very shortly all of my meds will be in a locked box with a combination lock.
FYI: did you know that adult multivitamins are the #1 cause of childhood death due to overdose? The iron, among other things, is incredibly toxic at an adult dose.
I might have your hubby present it as, I know you would never want to do anything to endanger the kids. Do you know how incredibly dangerous this is? If you appear "informative" instead of accusing it might go over better...
On a side note: I don't think I would let the kids have free reign of the house even if the grandparents start picking up the pills (though they should still be requested to do so). Ever cleaned out the car and found a french fry between the seats and realized you haven't allowed food in the car in months? Same thing with the pills, they could have rolled under the sofa or be hidden in a million places an adult would never think of and a child is certain to find.
Good luck,
S.