Also don’t forget the age component. If you have a first year in pre-calc /Calc, they are going to be with much older students. It can take them a while to get used to the pace of the class or the subtleties. So even if they have the math skills they might have to stretch. A lot.
Also, BS’s will test your child before they let them start well ahead of the normal course progression. An online class isn’t likely to cut it.
Many mathy kids can’t wait to work independently with the math dept. Make sure this is possible. Also check that a class will run even if they don’t have enough kids. If your kid wants to take linear algebra you don’t want them to wait a year for the course to run.