Tag-alongs don't give the kids the freedom they want when it's not needed. Pushing with your hands while alongside is also dangerous if one would swerve both can end up in a heap. Wires or ropes can get tangled in the wheels. There is really no solid once size fits all solution. Perhaps the least dangerous is a tow strap, like what you would use for a car. It has enough give to not jerk badly when starting out and is thicker than a rope or wire. Here's a Youtube video of one being used.
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Again, if the front person slows down there is danger of it getting caught in the spokes, for the front person this means a sudden stop, for the rear it could pitch them over the bars. You will just have to try it out and work out the rear person applying the brakes to keep the strap tight so you can sop and unhook.