When I started teaching my kids physics I kept having the thought "it'd be really helpful if they knew calculus". Then I thought "why would you teach calculus separately from physics?" So now that our 11 year old is ready for calculus, I'm teaching him calculus & physics together. (Here's some calculus, and here's how you use it to solve this physics problem.) It's going swimmingly. An hour a week goes a long way when you have a solid foundation and one on one instruction.