Will he need to adopt the children legally for them to travel along and can you please explain the full procedure?

Generally, a foreign national living abroad who is engaged to become married to a U.S. citizen may complete an immigration process that includes the foreign national's minor children. There are multiple approaches to be considered, including, for example, an application for a K-1 fiance visa that would allow the foreign national to enter the U.S. to become married within the required time and then proceed with adjustment of status; the couple could become married abroad and then complete the consular processing steps in the Immediate Relative visa category, etc. It would be wise for you and your fiance to consult with an immigration attorney who, after learning all of the relevant information, could advise about immigration eligibilities, options and strategies and who then could offer legal representation for the often quite complex application process.