The marital status and age of your children are important factors in the immigration process. Technically, your son/daughter can get married after you file Form I-130, Petition for Alien Relative, on their behalf. However, it will cause additional challenges that depend on your status as a petitioner.
If you are a US citizen and you file Form I-130 for your unmarried child who is under age 21, your case falls under “immediate relative category” (reserved for the spouse, unmarried children and parents of US citizens), thus your son/daughter can get a green card quite quickly. But after your child gets married your relationship gets classified as a “family preference category”, this will add few month or even few years to wait for the green card.
If you are a permanent resident filing Form I-130 for your unmarried child who is under age 21, your case gets classified as a “family preference category”. And if your son/daughter gets married after you have filed Form I-130, your relationship becomes ineligible for receiving a green card. Currently, there is no visa category stipulated for married children of permanent residents.