I lost my kid on the banana bikes in Dulwich Park - we were going round the pond then moved out to the outer circuit. Took seconds to lose them. Funny now, scary then. I don't think she even noticed, was having too much fun. Kids of course don't notice you're not there until they realise you're not there, which could be much later. Ride close behind them, stop frequently enough so they understand the next part of the journey and make sure they can hear you/ know to respond to you.