This is a collection of resources. Please post links you would recommend, add a brief description and a recommended age group. I will update the list at the top of the thread.
Scratch -
https://scratch.mit.edu/Scratch is a programming language and an online community where children can program and share interactive media such as stories, games, and animation with people from all over the world.
Flowlab -
http://flowlab.ioFlowlab is a game-making tool that's targeted toward beginners.
Unreal engine https://www.unrealengine.com/en-US/A 3d game engine where you can build a level using prefab objects and physics.