[baggsie]

Geometry and stiffness make the biggest difference to handling with weight being of negligible benefit (on a mountain bike a bit of extra weight can actually be beneficial on jumps). You and your son were probably simply riding better designed bikes.

[newaccount74]

Weight is pretty important for kids bikes. When your kid weighs 20kg, it makes a difference if their bike weighs 12kg or 8kg.

[TulliusCicero]

Well, that too, but I do think the weight makes a difference for acceleration.