The judge sets bail on the theory that the money amount needs to be high enough to insure the defendant does not flee the jurisdiction. The bail amount you mention may be pretty low, depending upon the economic circumstances of the family of the 13 year old child. The crime might be real minor, but if the chance of flight is high, the bail amount will increase.
I hope this helps you understand that severity of crime is but one small factor in the judge's discretion in setting a bail amount.
Bail is set after considering several factors including but not limited to the accused's prior record, the severity of the crime, and ties to the community. Your question identification says you are writing from Louisiana. As an example, you might consider the Jena 6 case in which bail for six teenagers accused of Attempted Second Degree Murder in Louisiana was set at varying amounts between $90,000 and $138,000. Typically, the highest bail amounts are reserved for the most violent crimes. Bail is not supposed to be punishment, but sometimes politics affects bail amounts too as in the Jena 6 case. It's important that the accused hire the best, most experienced criminal defense lawyer he can.