3 to 6 it's very young they can't read or write. If I were you I would teach them vocabulary using songs. Games are very useful for very young learners too. ( have a look at
www.mes-english.com ) there 're loads of games for young learners .And why not use Cambridge young learners papers
( the listening section ) for comprehension? The last part of the test is called : listen , colour and draw. And there's a conversation between kids , they have to tick the correct picture. I found it very useful with my younger students.
Good luck!
Michèle