The Shulchan Aruch 199:10 writes that a child can be included for a zimun of three or 10(only one minor is counted),the Rama does not allow a minor to join.
1) If there is one adult Sephardi and one adult Ashkenazi and a Sephardi minor who ate together can the Ashkenazi count the minor?
2)If there are 8 Sephardim and one Ashkenazi and one sephardi minor,does majority rule against the Ashkenazi(diff from case one where its is one against one).