What was the biggest mistake I made when I worked

When I just joined in my work,I didn’t know what I should care.Once after lunch I took the children to go for a walk.They lined up one after another and walked.Suddenly,I heard someone crying,so I looked for who was crying.Finally I found it was the last boy was crying.Then I asked him what was happened.He didn’t tell me and still cried.At that time,an older teacher came here and looked around the boy.Then she told me that he hit the door.She said :”Look!When he goes through the door,his head hit the door.There is a red bruises (不确定是不是能这样说)on his head.”At that moment ,I didn’t know what I should do.I was very nervous.That teacher gave me a towel with water and let me put it on his bruises.After while,the bruises on his head was not very red.
After class,When the boy’s mother took him away,I said sorry to her.That was my mistake.I didn’t take good care of her son.She was very nice.She said it doesn’t matter and didn’t care it. 
Through the accident,I know that I must look after every children.No matter where are they,I should look at them all the time.Don’t let them hurt and make them happy in the kindergarten.I won’t make these mistakes.This is my duty and work experience.

小细节稍加注意就是一篇还不错的小文了，继续加油~