MrsChizi

Then you need to petition her too. If you wait for him to come and then petition her, they will have several years apart. But as you're marrying before her 18th birthday and she'll be under 21 when you petition her, she'll be classed as your immediate relative and so get a visa in the same timescale as him if you do it. So it's two petitions (she can't be a derivative on his), but they'll each get green cards on entry. You don't mention the mother, but bear in mind if she is still alive you'll need her permission (or a court order) to remove the child from her home country. As for posting in the main forums, just go to the relevant sub forum and then hit the 'start new topic' button to create your own thread with any questions. See screenshot below. 

thank you for all this valuable information! why cant she be a derivative of his? is it because of her age?

There are no derivatives for immediate family members, each needs their own petition.