Did you marry your wife before her oldest child was 18? If not, you cannot file for that child.
Also be aware that you have a choice of (1) filing only one I-130, with your wife as the primary and the children as derivatives, or (2) file a separate I-130 for each child in addition to an I-130 for your wife.
If you married your wife before your green card was approved, you may also have option (3), follow-to-join which is likely to be much faster than (1) or (2), depending on the category of your green card. The same goes for your children, if they were born before your green card approval (but it may be more complicated for your wife's child).
Note that if you choose option (1), and become a US citizen before their green card are approved, they will lose eligibility to immigrate as derivatives and you will be forced file a new I-130 for each child, which could introduce delays. So if you plan to become a US citizen in the next couple of years, consider filing a separate I-130 from now so your citizenship won't disrupt their processing.