The question relates to the children being biological children of the US Citizen petitioner. If the US Citizen petitioner was a USC at the time of each child'e birth and meets the physical presence requirement, they don't need visas. They need Consular Reports of Birth Abroad, and US Passports. Please clarify whether the petitioner was a US Citizen when each child was born, and whether they lived in the USA at least 5 years, one year of which was after turning age 14. If so, they are US Citizens and cannot be issued immigrant visas. You take the CRBA, and US Passport route.