The straight strips should work well and then cut for Rail Fence or something similar. The older ones might like to do the Log Cabin or Courthouse Steps. Still strips but a little more interesting. For those who understand the concept a bargello would be fun at any age.
I've never done 4-H so didn't know the adult requirements. I did Girl Scouts and the younger children do need more one on one than the older ones.
Depending on their interest maybe you could consider a little information on quilt history, too.