We have 5mg tablets here.  Are you in the UK?  That’s about the only place I know of where they only do 10mg.  That’s why Ashley ended up on 5mg x2 a day when a baby and switched from suspension.  I presume that they are oval pills and I wondered if you have tried quartering them with a pill splitter?  Mind you if you have 5mg at least you could split to increments of 1.25mg.  

If all I could get was 10mg I would write to the maufacturers and tell them to get their act together and do my best to split to increments of 2.50mg with what I had until they did that.

http://www.cahsupportforum.com/cah-tablets.html

Cheers