So if my kids are trouble and they need to contact me, your suggestion is that they call me at the concert or venue.
Yes, that's exactly how we did it in the old days before mobile phones. Name and phone number of the venue posted by the kitchen phone. If the kids aren't capable of making that call then they should have a babysitter.
Then there is the problem of the helpful "staff" as you call them finding me in a sea of other people.
Pause the show, make announcement over the PA system. Remember, this is in case of emergency.