The idea is a good one, but you will have trouble making it happen. Being a teacher, you know preschools and grade schools have waivers that they give to parents to sign, allowing the school to take pictures of their child and to use it in newsletters, websites, etc. The people taking the photos are the teachers and staff at the school. You can't walk into a school where nobody knows you and ask to start taking photos. Even if you have a company, the school doesn't know how legitimate that company is, and what you will do with the negatives. Besides issues of pornography, there are issues with divorce and custody battles in families.
You might have better luck talking to people you know and taking pictures of their kids. Word of mouth would be your best advertising.