Interesting…
When we started collecting hatching eggs, we put these particular hens in a breeding cage with the rooster for a few days, then after that they went straight into dog crates until they laid their egg each day. After an egg was laid, we let them out with the hens for the rest of the day, and put them in the crate again the next morning.
We do have barred rock hens…but all of our roosters live in a bachelor pad, separate from the hens —so even if my daughter did accidentally put an egg from a barred rock into the hatching eggs, it shouldn’t have been fertile.
I’m trying to think back if we had any incidents where the wrong rooster may have gotten ahold of these hens

…we did have a blue australorp get in with the wrong rooster right when we started breeding…but we stopped collecting her eggs — I wonder if we stopped collecting eggs of the wrong blue australorp?
Would a blue australorp crossed with any of these breeds produce this chick: Leghorn, California White, Buff Orpington, New Hampshire?
If the answer to that is “no” then I guess we have to assume that at some point we had an escapee rooster that I’m not aware of, he got the barred rock hens bred, and my daughter accidentally put one of those eggs in the hatching container…