I am not sure how close you and your children are, but there might be something your children are not telling you: their problems at school. Of course, sooner or later you are probably going to discover that something is wrong just by looking at their grades, but it would be so much better to take action before any problems with studying subjects such as math occur.
From my personal experience I know that many children experience difficulties while learning math including calculus. It is no wonder then that online help for ap calculus can be of such value. I myself remember being tutored at math when I was younger, not because I was very bad at math, which I wasn’t, but because I wanted to get to university that required me to have advanced mathematics skills. In my case, the tutored helped me to get to the university of my dreams, something I wouldn’t have been able to do without his invaluable help.
If your child has problems with maths, I suggest that you approach his problem carefully. Don’t criticize your child for having problems with math. Many children, if not most of them have problems with math at some stage of their lives and it is perfectly normal for such problems to occur.