What are some good platforms/apps for kids to learn coding?

This is good one to start. It is actually a library for building visual programming editors. But on it's page you see other projects built using it, which you can use.

This is a good option for students provided that they have access to an ipad. This app teaches Swift code concepts and fundamentals through puzzles and simple challenges. They recently added advance skills that allow you to control robots and too.

You know why.

Coding and design courses for kids and teens ages 10 to 18. Python, iOS, Java, Minecraft, 3D printing…it’s all here! Each course contains lessons, support materials, and interactive challenges.

As their Tagline says "Anybody can learn to code", they created courses for every age group, its nonprofit organization where anybody can learn or teach.

With ScratchJr, young children (ages 5-7) can program their own interactive stories and games. In the process, they learn to solve problems, design projects, and express themselves creatively on the computer.