Hi Steve
I set up my Code Club about a year and a half ago, Started off initially with year 5 and 6 with one boy from year 4 who was keen to join. Then over time we let in more year 4 and then some year 3. This year because numbers were getting quite high at the end of last year I decided to restrict it to years 4 to 6. So at the moment I've got 8 from year 4, 8 from year 5 and 2 from year 6.
I actually find it is easier to keep the younger newer members focussed on the more proscriptive Scratch projects. They tend to stay "on script" more. Being able to use Scratch 1.4 helps them focus more whilst Scratch 2.0 online makes it easier for them to get side tracked by other peoples projects (i.e. play games!). But a lot of the projects are designed for Scratch 2. The older children that have already done a lot of the Scratch projects can get bored so I find I have to look for challenges to interest them. The other day I got three of the doing some Python tutorials on Code Academy. Felt good getting them writing "real code" :-)
The other big challenge with a mixed group, especially if some have been with the club for a while, is that you end up with the children doing a wide variety of things which is "interesting" if you are doing it on your own.. But it's fun!
Doug