For smaller rabbits you can use the Excel Dwarf and Junior pellets, which are specifically small-sized for little buns.
However, the fibre content is only 18% in these, as opposed to a more appropriate 28% from Oxbow pellets, so I am a little torn between the two. I think as long as the buns get lots of hay, the small, slightly lower fibre, pellets could be a good option for little bunny mouths?
Jen