If money cannot buy happiness, it also cannot buy intelligence. Parents desperately try to convince their children that school is important and that they need good grades thus they resort to rewarding them. It is my strong belief that children should not be paid for getting good grades at school for several reasons.
First of all, good grades do not equal intelligence. Parents aim to teach their children the purpose of their homework and the latter will only do them for money. All parents wish for brilliant children, although for example if a child strongly dislikes a certain subject and gets a good grade at it for money, the satisfaction he will receive will be absent and he will expect higher and higher rewards for getting good grades on that subject. Parents need to let children grow on their own sometimes and embrace them as they are, even if they fail in life.
Second of all, receiving money for good grades will give children a false sense of acuity and they might become arrogant. Encouraging words and help are better than money. Over time, the child will understand why learning is important and he will start to enjoy learning and therefore he will learn for himself, his mind, and his soul, not for pocket-money. 
Admittedly, randomly rewarding a child from time to time can turn him into an eager beaver. After working hard and being surprised with a reward, the child will do even better next time as he learned for himself, also receiving money. 
In conclusion, children should invest in themselves, rather than work for money. School prepares them for the real life, therefore they should not be rewarded, they should be guided by their parents to learn to enjoy what they’re doing and to always keep the good work up.