We do that at our church and ask the parents to give us a cell # if we don't have it. I think that it's a reasonably safe bet for most people that they have a cell phone. If they don't have a cell phone, we ask where they'll be so we can find them if needed. Our pager system went out a while ago and the cost to replace was hard to justify.
I know that some worship presentation software has a way to put up a number on the screen. That could work as well for parents who don't have a cell phone and will be in the main service. If that number/code appears in the corner of the screen, it's a note indicating that their presence is needed.
I go to a much smaller church, so we're pretty much able to keep things personal, but I realize this could be a challenge with larger campuses.