The Tashi books and the Toys Go Out series are both good transition series. They have bigger print and wider spacing, and the Tashi ones have cartoony pictures on many pages.

We also like the magazines from Cricket-- Muse is a must-read around here, Cobblestone is also quite good. They usually have narrower columns of text, which help.

I wouldn't push this. Some kids really aren't developmentally ready for smaller print/denser text at this age-- some are.