Don't think there is a right or wrong answer here. It will depend on firm, what you're trying to show/how you use them, type of application format, and so on. Key is to draw skills and experience out of the mini-pupillage
I would suggest going one mini-pupillage at a time and treat it like a VS or job. List out the responsibilities, skills, tasks, what you learnt, etc and a line if appropriate how it made you think about covering to the TC route. There's a danger grouping them together you lose the skills and experience. Try and show you've learnt something from each one.
Cover letter, perhaps treat them as a group and highlight skills/experience and how it's made you want to change path.
Some applications will only give you 4 spaces and then an other info section/box (I think CVmail from memory do this). So it depends on what else you have to include here in the work experience section.
CV, split out chronologically.
Hope that helps. It's just how I would do it, but i'm sure open to other comments.
