She didn't guess them. She obviously watched you type it in and copied you. Hopefully, the messages were disgusting enough to be their own punishment.

Yep. Lots of people put in passwords in front of people ALL THE TIME without paying attention. I always intentionally look away to be polite but it happens in front of me a lot. Be more careful, get a privacy screen (anyone looking at the phone from the side won’t be able to see anything), and/or switch to fingerprint/Face ID. I don’t want to say YDI for underestimating the power of children’s nosiness, but....

You're all wrong. Is there any chance the child unlocked the phone by pointing at moms face? Keep working, though. You'll get your IQ out of double digits

A determined child can unlock a password very quickly. Might I recommend getting them started in hacking, coding, and other security training? Play your cards right and she might have a lucrative career as a security tech ahead of her...