I've towed two kids (both under 5) in a trailer on the back of my streetmachine without issue. Actually, I've done the same once on a quetzal recumbent tandem. No stoker, so me, plus 32kgs of bike, plus 20 kgs of daughter, plus 25 kgs of son, plus the trailer, plus no end of gubbins you have to take with you when you have kids. It was fine. Stable. Lovely. Aerodynamic in just the way that a brick isn't.