Parenting faux pas

I am so very thankful that Miss Girl acted out of character tonight! She is usually a mother hen, who will get others to do what they are supposed to do, even if it means physically guiding them....
I heard the toilet flush in their bathroom, after she'd already been in bed two hours, then I heard the soft patter of little feet coming toward my room. She peered through the open door and whispered loudly, "daddy, the toys didnt go night-night."
She then grabbed my hand, and dragged me to the bathroom, where the bath is still occupied with water and toys. Lil Miss had started crying during Girls bath, so we finished up, and I took her with me to go tend to the baby, forgetting about the bath....
Thank God she didnt try to get the toys out herself, because they have trouble getting in and out of the bathtub, and God forbid she'd slipped and hit her head, and possibly fallen into the water.....would I have heard it?

Lil Miss is officially 6mo old. Its bittersweet, in that my first daughter never made it to that milestone, passing away at 5m27d. (SIDS) Ive been a bit more anxious as that milestone grew nearer, as I was when the twins neared it. Its sometimes easy to forget that kids aren't invincible, even as I sit here now, with a 6mo, my mother hen safely back in bed, and my son recovering from heart surgery in the hospital.
The one child who I didn't think I had to worry about, ended up being the one that could have been seriously injured, or worse!

Im going to try not to beat myself up over this, but it does reiterate how important it is to not only teach boundaries, but also to keep aware of your environment, because teaching doesn't always work.
We have taught the twins over and over that they are not to be in the bathtub (or pool, pond, lake, deep puddle, ocean, etc) without an adult there. But they don't always listen to rules, and that could have very easily been the case.
Or, had this happened a few months down the road, it could have been Lil Miss, reaching for one of the duckies.

My nightly check includes securing everything in the kitchen, and all the downstairs windows and doors (windows are usually open during the day, because of our temperate weather), the lights, the windows in the kids room and nursery (putting the twins sippy cups in the mini-fridge, as well) and quick overview of the rooms (make sure monitor is working, nothing is in the crib, or near it, make sure twins haven't stashed anything, etc). The bathroom wasn't something that occurred to me to check, since it has no windows. But, alas, my Parent-brain kicks in. Forgetting to drain the bathtub. I will be including it for now on.
Being a parent is like working in the IT field. You have to constantly be upping your game, or you risk mild-to-severe consequences. You cant still be trying to baby-pad the brick fireplace when your kid is now running out into traffic. You have to stay ahead of the game, or at least keep up!