Americans traveling as tourists to China from Japan

We are traveling extensively around Asia and will me in Tokyo for two weeks before going to China. We have an invitation letter and all the details for our 8-day stay in China. However, we heard we need our kids birth certificates (a copy) to apply for the visa, which we don't have . Is that a requirement? They have their own passports. And, where would we go in a tokyo to apply for it? The Embassy directly? Thank you!