The problem with rewarding for grades is that you have two children who may be completely different when it comes to school and grades. My oldest (8th grade) has never had to work hard for her grades, A honor roll was a given every six weeks (it has since become a greater challange, thankfully!)
My younger dgtr (4th grade) is dyslexic and has done a fantastic job overcoming her disability to achieve in the classroom, but for her "achievement" is most frequently in the B range. If I were to only award her A's, it would greatly diminish her hard work and efforts she had to put into reading and spelling just to PASS language arts. She does not receive any special treatment for her dyslexia (per our request) so she works HARD and deserves to be praised.
I was the oldest in our family and breezed through school. My parents paid us for grades, so when report cards were handed out I collected up to 100 dollars by high school while my sister only got about 20. As a result, she hated school and barely graduated. That was only a part of what resulted in a great deal of sibling rivalry (we've thankfully overcome). You don't want to set your beautiful children up for that kind of relationship.
Hopefully, your dgtr will be A honor roll material just like your son. Just don't start something now you can't easily back out of in the future. Make it a celebration of how wonderful your whole family is doing, including yours and your husbands small victories! It will instill family pride and a sense of how everyone does their very best because that is what your family values and celebrates.
I didn't mean to sound preachy, our report cards just came out yesterday, so this is fresh on my mind and in my heart.
Just to brag, 4th grader made an A in reading this 6 weeks! But, sister made an 89 in math, we celebrate anyway!