This is the 16222nd most frequent Polish word.
upominać
"Upominać" means "to admonish" or "to remind."
In this sentence, 'upomina' is used to indicate giving advice or a reprimand gently to the child.
Matka upomina dziecko, żeby nie biegało po domu.
The mother is admonishing the child not to run in the house.
Here, 'upominam się' refers to asserting or demanding one's rights or entitlements.
Upominam się o swoje prawa w pracy.
I assert my rights at work.
In this case, 'upomniany' is used passively to describe having received a formal reprimand for an action.