1) tell the priest
2) if priest does nothing, tell his superior
3) if neither does anything, then you've done all that's required of you, but if it's an occasion of sin for you, find some way to mitigate it, either by sitting all the way in the front, or on the other side of the church, or even, if necessary stay out in a vestibule area and go in only for Holy Communion, etc. If it persists, and it causes sin, make sure you bring it up to that very priest in Confession, suggestion that his inaction had led to the sin ... and, if it's truly a near occasion, then you should find somewhere else to go to Mass
Some of this could be mitigate by returning to the custom of having men on one side of the church, and women on the other. Of course, there's this modern sensibility against separating families, but the Mass is primarily about worshipping God. Younger children could sit near their mothers, while the older boys would go with their fathers. That's the primary reason Our Lord was separated from Our Lady and St. Joseph at the temple. So the occasion for the trip was because Our Lord had come of age. Prior to that time, children would be with their mothers, but then when they came of age, they would be with their fathers, learning their trade or whatnot. When people traveled back then, they often went in caravans, larger groups, to prevent being attacked by robbers for whom a couple people alone would be easy prey, and when they traveled in groups, the men often were with the men, and the women with the women. So St. Joseph concluded that Our Lord was with Our Lady, as He had been accustomed to doing, whereas Our Lady assumed that He had gone with St. Joseph, now that He had come of age. That is why Our Lord's response was that He was doing His Father's business, since that's precisely what would be expected, that a young man at that age would beging to undertake his father's business.