Young children don’t ever believe or conceive of the notion that an adult’s anger or frustration is anything other than justified towards them.
A child cannot comprehend that you are having a bad day and are irritable. Or have a headache and are extra snappy (unless you let them know and they are in turn capable of understanding). They believe your anger has something to do with them, since you are right with them; and ultimately they may arrive at the conclusion that they are the cause. They accept that they must be bad. Because why would you lie? Your face and body and tone don't lie. You need not say a single word for a child to believe this about themselves.
Everything about your world together, from the very beginning has been one of your child looking towards you for direction; understanding, comfort, unity. You are their compass point and source of sustenance. Before they discern that they are a separate self to you, they were also you. This is also physically correct.
If your emotions are as clear to them (more so when words are only just beginning to make sense) as anything else they experience it means that you cannot hide or lie! No frothing and seething with teeth clenched, forcing a false smile.
So what can you do? Must you never get angry or frustrated! Of course that’s not a reasonable expectation to place on yourself.
Far easier to be open with a child, tell them, no matter how young, that you are cross about forgetting something or about getting a parking ticket, that a friend forgot something important…. Let them know you are not angry with them! You know best how to communicate this to your child. Trust yourself.
Better still take a moment of parental time out to breathe. To collect yourself and let go of anything that has previously happened in the day so far – it’s gone! You cannot change it now! Temporarily release all things on your to-do list in the day ahead of you. The moment is now. Focus on connecting with your child and reminding them that they are awesome and loved. That sometimes people get angry.
Release and move on.
Still connected.