Visa or travel document for my children to visit Shanghai, China

My wife and I are still holding Chinese passports.
We have three children, two daughters and one younger son, all were born in America. They all have american passports too. We planned to take them all back to Shanghai to visit their grandparents in May 2015.

Two daughters were born before we got green cards, they both got travel documents when 1 and 3 years old, visited back to China then. My son was born after we got green cards.

My understanding is that we should apply for Chinese entry permit for my son.
But I am not sure for my daughters. I contacted a travel agency that help handling the application service. I was told since my daughters were born BEFORE we got green cards, they must apply for travel documents again.

Can you give me a clear answer?