Sometimes the best way to deal with 'silly' rules is to stick to them rigidly and with lots of publicity, exactly as done here. That way the senior management get to review things and either allow a bit of discretion or modify things to work better next time.
Subject to what the lawyers say, provided both the parent and the helper understand the risks and agree to them, the airline should just let them get on with it (they're still safer than they were on the car journey to the airport). After all, had the mother found someone in the departure lounge who would agree to help out, would the airline have been any wiser? Two people boarding together with two children wouldn't have attracted any particular notice