what should be the ideal weight of kids school bags ?

any load on the shoulders will, in the best scenario, overcompress intervertebral discs, impede the correct posture and compress circulatory vessels, I would advise to use a trolley or, at least, carry the bag by hands or hold it with arms, to minimize damages; I also advise to start stretching since youth.

Ideally no more than 10% of body weight.

An adult can carry up to 20% safely but not recommended for growing child.

15-20 pounds..

0-5lbs. 

Ideally. 

That would depend a lot on the age, size and strength of the kid, but I have to tell you that I know a tall, strong 9 year old boy whose school bag is way too heavy! Schools are using more online materials now, but there are still some heavy textbooks, and a lot of them.