It's OK I said, this is much simpler than it sounds. Just smile and say hello as you pass – treat young people with the same courtesy you would an adult. If you get to know children as they grow up, then when you catch them doing something silly and ask them to stop, they'll probably say sorry. If the first time you speak to them is to tell them off, they may not be so polite.
Despite the positive reception at the meeting, I left saddened. Something has gone badly wrong in the UK when decent citizens worry that they will be treated with suspicion if they talk to a youngster.