To find limitation

I have been baby sitting every other Saturday, and I am enjoying spending time with them.
They are a Chinese boy (9) and girl (7).

We went to a playground near my house.
A little black boy who seems to be 4 or 5 years old, kept saying to my boy and girl "Chinese Chinese ........"

My girl said to him "Stop it!"
My boy said to her "Just ignore him."
The second time when she said "Stop it!"
I could not ignore it anymore.

I said to him "Where is your parent?"
I heard from a bench "Yes I am his mother."

I said to her "He keeps saying Chinese Chinese, what kind of education are you giving?"
She was so upset and shouted at me.
"DO not attack me, I am not racist."
I could not talk any more and left.

Since I am Japanese, I usually could ignnore.
But I had to protect my children whom I was taking care of.
But I should talk more humbly and in a polite way.

Even if I speak Truth, it is judgment without love.
If I would attack, they will attack back.
I felt bad I couldn't set a good sample to my children.
Self control is not easy.
I appreciate that I found my limitation.