When they're really young, I hear they tend to take things literally, so it's more fun to threaten them to flush them down the toilet, vacuum them up, pull the plug whilst they are in the tub so they go down with the water, explode an egg in the microwave, and tell them they're next if they don't behave, but I came up with a really good one. Sibling is at a sleepover, so you had this all planned out. Pour some original V-8 into an unmarked bottle, and tell the bloody brat you put her sister through the food processor as you're pouring yourself a glass, and comment on how tasty she is, along with a 'Want some?'.

Then we have:

https://www.n1m.com/haarprecording

Scroll down to "A Good Career In Babysitting".

Of course the little bastich may require therapy for life after you're through with him/her/it, but hey! Have all of the incidents secretly filmed, and bitchute 'em, huh? I mean, I didn't hate myself enough to breed; so hey! Doesn't mean I can't enjoy watching someone else's 18 year gaol sentence put through the ringer.

Well, I did actually threaten to flush a 4 year old down the toilet if he touched our musical equipment, and he believed me. I'm still laughing over that and consider it a high point in my life.