Complex sentence: Clackers were eventually discontinued after children got injured while playing with them: besides being fairly heavy and fast-moving, the spheres, being made of hard acrylic plastic, could shatter upon striking each other, presenting a safety hazard.
Split into simpler sentences: Clackers were eventually discontinued after children got injured while playing with them. Although they were fairly heavy and fast-moving (the spheres, being made of hard acrylic plastic), they could shatter upon striking each other, presenting a safety hazard.