If you kids love lego they will love this application. It is quite easy once they get hang of it and today generation are quite good at learning new applications. It is fun and they will learn new things too. But make sure you limit the time how much they can play on this, as time flies.
LeoCAD has an intuitive interface that is designed to allow new users to start creating new models without having to spend too much time learning the application. At the same time, it has a rich feature set that enables experienced users to build models by using techniques that are more advanced.
Native versions are available for Windows, Linux and macOS to ensure that users are familiar with the program interface. You can download it from https://www.leocad.org/download.html.
LeoCAD is open-source, so anyone can contribute with fixes and features, and it is always going to remain free.