This is my formula for kids bows:
If you make a bow's weight about equal to the child's age (girls), you're pretty safe. After age twelve, a child's strength starts to increase dramatically, so I add 5 pounds to the draw weight. After age 15, I add 10 pounds to the draw weight. So....for example:
Age 5 = 5# bow
Age 12 = 12# bow
Age 13 = 18# bow
Age 16 = 26# bow
These examples are for girls, using my daughters as a guide. My boys are only 5 (twins) and I'm sure they will be stronger than the girls, but for now the chart above works well for them too.
As far as draw length, their age + 10 will determine the number of inches. Examples: a 5 year old can pull 15" and an 18 year old should be able to handle 28".