If Scratch is too advanced for the youngest of kids, try ScratchJr: http://www.scratchjr.org/
The kids also really liked Lightbot. Unfortunately, the web version uses Flash which means it won't run nowadays on most computers. There are apps though, including a LightBotJr version for 4+.
Here's a google doc with several other elementary school coding & robotics resources, although I haven't updated it in a while: https://docs.google.com/document/d/1r1b2CM1uTdST47IbWa7zlZYm...
The first time I had this course, the kids were between 8 and 10. The differences in cognitive behaviour were minimal. Just some of them were a little bit slower than the others, but at the end they helped each other and everyone was finished at time. I see, 4/5yrs would be much trickier.
For our next course it will be a little bit more "diverse". The oldest application is 14, and some of the former kids are coming again. So it will be the whole spectrum 8-14. I will see in the first lessons how I can separate them or offer different materials.
This time I'll offer some intro to Python. Maybe, one or the other will then grab this path.
I admit preparation of the materials was quit time consuming... more as I thought in the beginning it would be. But for the second course, I have already a bunch of prepared stuff.
you could try to approach with a single theme? I.e. I heard from talk by Felienne Hermans, that her code club used alot of sprites/images from the Donald Duck comics, as everybody was reading that, and then you could scale by ability?
I.e. younger kids could do just animations, older kids interactions?