Generally, it is MUCH easier if the children come along with the parent. There are so many issues to consider in each persons case:
-- How old is the child?
-- Does she have her own international passport, or is she listed in her mother's?
-- Is her father in the picture? Could he cause trouble about his daughter's travel?
All of these questions come in play as it could be several months between marriage (when she can file for AoS) and being able to travel back to see her child (receiving Advance Parole). And certainly several months will take place between filing for her daughter's K-2 and the daughter traveling to the USA.
Another issue is cost. Unless you like writing checks to the USCIS, you will be paying for everything twice. If done at the same time, you can file the K-1 and all the K-2s together for one application fee (plus biometrics fee for each person). And again when it is time to file Adjustment of Status. A several hundred dollar savings.