If your child is interested in engineering, and won't be able to go to a private school, it is best to have Calc I and II done before they go to college. At state schools, Calc is typically a weed-out class with huge lectures and they want a certain percentage of students to not be successful because these classes serve as a gating function to the engineering school. Combined with other freshman semester distractions and adjustment it's a good idea to have it out of the way.

Other than that I'd say the best acceleration is enrichment. Teach stuff like proofs, "how to solve it" by Polya, gain intuition about linear algebra and complex numbers. Martin Gardner and AK Dewdney articles in old Scientific American magazines.