You're very welcome, kgeaux. It was a solution I had come up with when the baby was first starting to climb the stairs. I was doing some dishes while she was playing on the floor behind me. My husband had come into the kitchen, and then left again. The baby was also gone so I had assumed that he took her. Meanwhile, he assumed she was still playing on the floor behind me. In reality, she was almost at the top of the steps. Thank God she didn't get hurt. It was at that point that I imposed my new "system" on everyone - lol. I'm very glad to share it if it can help!
P.S. When we first started doing this, if my son was playing with the baby, and she crawled off to a room where my older daughter was, he would simply yell to my older daughter that the baby was in there, and that she had to watch her. BUT he didn't bother to get a response! So I had to explain to him that you MUST get verification from the other person that THEY KNOW that they are now watching the baby or it does no good. I think we've worked out the kinks now.