It's not harmful if you swallow a penny.
However as an adult you really should not put any pennies or anything in your mouth that you should not do.
So it's not good to purposely swallow a penny or other coins as they may eventually cause some health issues.
But if a toddler or small child happens to swallow a penny or other coin and they are not choking then there's nothing you need to do.
I swallowed a couple of pennies before when I was 3 and then again when I was 4 years old.
My parents would leave pennies and other coins out on our end table and I would go up to the end table when I was 3 and 4 years old and grab those pennies and put one in my mouth and then before my mom or dad could remove it I swallowed it.
My mom called the doctor and they said if I'm breathing that there was nothing to worry about.
So make sure you keep coins and other small objects away from kids when they put things in their mouths.
Eventually the penny will come out in the toddlers or child's poop.