I would work with your child on problems. Do them as well. Then go over the mistakes. With two people, it’s a matter of the person getting the problem wrong explaining what they did to the other person. It becomes clear very quickly if the problem is carelessness or not getting the concept.
That said, stepping down a level may be a good idea. Sihe may need the extra year (it really depends on her errors). I’ve seen good but not stellar students crash in math in 11th and 12th grades when they have a weaker understanding and poor instruction.
Also if she shows that she is having problems keeping up with all her academic responsibilities, it’s okay to ease up the rigor on one subject. If math isn’t her thing, it may be the one subject.
I echo Mathyone - take away the calculator.