It is better for the kids to understand English before going to school. Children can switch languages without difficulty. However, if they don't know English at all, most likely, they will have the wrong impression that speaking Chinese is wrong. Since when they say certain things in Chinese at school, teachers and their classmates will likely to correct them without telling the kids that their answers are correct, but in different language. Your accent will have little impact on your children. don't worry about it.