I don't think the kids did know. But dad was probably giving them vitamin pills not just for the nutrition, but to condition them to swallow whatever he gave them, so they would not question him when he poisoned them.

Very sick in the head, when there are options and alternatives.

 

When we first see the family I was not at all sure that the Kids were their Kids at all?
I thought they got left behind somehow and they were just taking care of them especially with the callousness of the "Power Pill" comment from the boy and how cold the Father seemed to them both at first?

I am just not feeling it here.

 

The kid didn't know about the suicide plan, I'm sure the father just talked to him about the pills to prepare him for the moment when they needed to use them. The kid thinks they will just keep his family together and keep them safe. He will willingly take them when the time comes. The girl was surely told something similar which made her curious to take them after she saw where they were kept after walking in on Nick.