I seem to remember one of the standard lines in a baby blessing is something along the lines of ",,,and the name by which you shall be known on the records of the church is...."
All the membership growth numbers released by TSCC don't make sense. Others have suggested that children of record are currently counted, since the math is so seriously messed up, but I would send resignation letters for them.
None of my three kids were baptized, but all were blessed. Just this last week, members of the church have come by the house two different times (at 8:30 at night) to ask about my two oldest kids who are 24 and 21. I told them they weren't interested in the church and to save themselves a trip concerning my youngest son (19) because he wasn't interested either.
They come around every few years to check up on them. My daughter has sent in her resignation through email but has heard nothing back yet.
Incidentally, a few years ago when she was living away from home, the missionaries randomly knocked at her door. She saw who it was and decided to answer the door in her bra. She thought it would scare them away. I told her that I was surprised that several more missionaries didn't show up at her door after that. haha
Anyway, if you don't want your kids to be bothered for the rest of their lives, have them resign.
My kids were not blessed or baptized and they have never been contacted, i think they go on some kind of list if they have been blessed but not dunked.
and as such show up on the membership records of the Ward in which you live, just as a baptized member would. So to get them off the records you need to send in resignation letters for them.