No reason to provide. If the kid is yours, he/she is eligible to apply for H4 and your kid can stay here with you same period as your H1B Max period. Documentation your kid needs will be:
1. Your I-797 (origianl)
2. Eemployment verification letter from you employer
3. Your latest pay stabs
4. Your kid's birth certificate or related documentation to show your relationship as parent and child
5. Photos to support the kid is yours.
Remember that your kid's H4 application can not be rejected by 214B. The only evidence you need to show is the relationship between you and your kid and your status in USA and you have enough financial ability to support your kid, not to be the public burden here. Your kid can go to school and it is free if the kid goes to public school.