Another Mass ruined by a selfish idiot, who caused me to commit several sins just thinking about the noise his/her child made throughout the entire Mass.
The situation is different if there is one poor parent (usually the mom) attending Mass with more than one child. If one child is being loud, in this case, it's hard for the parent to leave without rounding up all the kids and creating even more of a ruckus.
However, in every case of scream-itis I have seen, either two parents are present, or there are only one or two children. In this situation, it's extremely easy for one parent to get up with the screaming child and leave. Why they don't do this, but rather sit there, as if dead, while the child climbs over them, banging on the pew, etc, I will never understand.
Others have made the point that if one says something to the parent involved, that person may leave the parish and even the Church. What of all the other parishioners who leave the parish, and even the Church, seeking a community of respectful reverent people who make an effort to silence their children in God's house?