Scratch is a great tool to help children learn to code. It is designed for 8-16 year old, but younger children really enjoy making simple animations with the program. It can be downloaded to a computer, tablet or iPad or used online. There is also a Scratch Junior app for children in KS1.
Below are links to the download and also lots of projects to get children started.
The below games are for an earlier version of Scratch but will work on Scratch 3