U.S. citizens can apply for an IR-2 visa for their stepchild/stepchildren much the same as they can for a biological child. The caveat is that the child's birth parent and stepparent must have been married before the child turned 18. The stepparent does not need to formally adopt the stepchild in order to apply. If the stepchild is under 21 and unmarried, they qualify as an "immediate relative" of their American citizen stepparent. This means they can apply for an IR2 visa without waiting for a green card to become available. If the stepchild is married or over 21, they don't qualify as an immediate relative, and will have to wait for a visa to become available. This process usually takes a number of years.