If you’re referring to the shiny pieces in soil you buy from a garden shop, that’s actually perlite - a natural volcanic glass that’s added to improve soil drainage.
Counterpoint- I have three children (one still not biking in his own) and have exclusively used a cargo bike to cart them to school since Kinder unless it was exceptionally icy roads.
That said, I live in a very bike friendly city now, close to their school, can afford a cargo bike, etc.