AP study books, Barron’s is a popular title but use any AP study book you find that you like.
You might have local private schools and organizations that prepare children for the AP exam. Even though my children attend a Sunday-only Chinese school program at a local church, I know that there are children who attend the school who go on to take and pass the AP exam. There are many small schools dedicated to preserving language and culture, such as L’Ecole Française de Cincinnati, which specifically says that it prepares children to pass the AP French exam. Many, probably most, of these schools are intended for children who have at least one parent who speaks the second language. But, if you’re interested in having your children attend, then ask anyway. If your children are happy to attend, the school might be happy to have them.