Double trouble

We've just been to a music program (concert) in which 13-yo was performing. One of the songs that the eighth grade choir sang was from the witches in Macbeth.

"Double, double, toil and trouble
Fire burn, and cauldron bubble."

When we got home, 6-yo was asking me about it, wanting to know what the words were.

"Why were the witches singing about toilet trouble?" she wondered.

Talking of phrases which get you wondering, she and 9-yo often say "on accident" instead of "by accident". Husband and I have started correcting them. Up till now, it's been one of those family phrases that we fondly think is rather sweet, but the time comes when the need for correct parlance trumps parental doting. It's been a very hard habit to break. Husband and I assumed that they were saying "on accident" because of the parallel with "on purpose", or possibly because "it was an accident" could be construed as "it was on accident". Either way, it sounds odd to us, and we just thought it was wrong usage.

But... in the past couple of weeks, I've heard "on accident" three times - on one occasion it was a child, but on the next two, it was adults (albeit young things).

Is this becoming current usage? Is it an Americanism, or have you heard it over the other side of the Pond?