as per some versions she creates Ganesha out of loneliness-its said Mahadev was absent for a long time,its then that it happened!
so most likely Mahadevs going to go with the Sanatha Kumars for some teaching(since they belive he is thier guru)
so then Parvati will create Ganesha-uske baad the rest of the story is the same-she asks him to guard when she goes to have a bath and then everyone knows the rest
or it could go the other way too-that Parvati is miffed with Mahadevs aloof behaviour and creates her own son