About 10 years ago I was teaching a group of 8 kids from an elementary school (4th year, aged 9-10). We've been using the RCX Mindstorms sets.
This project was funded by the Hessian "Kultusministerium" (Ministry of School, Education, and Culture) for highly gifted pupils.
We had 4 Mindstroms sets, so 1 Mindstorms set for 2 kids each - this is the best ratio to my experience.
For the first 3 lessons we were 4 adults (me and 3 assistants), afterwards I was teaching the group alone (with 1 optional assistant if kids have been missing a lesson).
But I must say, I honestly doubt that kids younger than 9-10 are the right target group for robot programming at schools.
NXC CHESS for NXT: http://www.mindstormsforum.de/viewtopic.php?f=70&t=6790
indispensable for NXC + EV3-C: easy + trouble-free network & sensor+motor remote control for rs485, BT, USB!