The reason why parents put pennies on shoes is to improve the traction of the sole or bottom of the shoe.
The pennies that are glued onto the bottoms of the shoes or soles of the shoes help provide grip to the surface especially on slipper surfaces or smooth surfaces.
Pennies on the bottoms of kids shoes also can provide a tapping sound that can help parents hear and track the child's location when they are close by.
Gluing pennies to bottoms of shoes can also make some normal shoes into some fun tap dancing shoes.
Another reason for parents gluing pennies to the child's shoes is to keep the child from distractions when walking,
The tap-tap sound that the pennies make when walking with them glued to the bottoms of the shoes also can keep the child alert while they are walking instead of diverting the child's attention to something else.
I used to glue pennies to my own shoes as a kid when I needed extra traction and it does really help.