Some parents handle the situation well; some parents have meltdowns of their own.
As for the bystanders, usually those who have recently had children at that age are very understanding, while some others (especially those who have never had children or worked with children) are not.
In today’s Gospel (Matthew 18:1-5, 10, 12-14), our Lord and Savior Jesus Christ says many wonderful and beautiful things about children.
Amen, I say to you,
unless you turn and become like children,
you will not enter the Kingdom of heaven.
Whoever becomes humble like this child
is the greatest in the Kingdom of heaven.
And whoever receives one child such as this in my name
See that you do not despise one of these little ones,
for I say to you
that their angels in heaven
always look upon the face of my heavenly Father.
So it is for the children when they are peaceful, loving, trustful, and innocent.
So it is also for the tantrum-throwing toddlers.
God, the most loving and understanding of parents, understands.
He understands and loves even the toddler who screams and cries like a wounded banshee in the middle of the supermarket.
He understands and loves even you and me.