This is the age group I work with. Don't try to be too ambitious. Working on voice projection is way too advanced at this age when many of them will be too shy to speak at all. Keep everything fun and use games to achieve objectives. If you want to do some voice work I suggest some fun tongue twisters and copying sounds, nar nar nar nar nar in a witchy like voice is good for incoropratng some twang into the voice which does help projection. Everything will need to be teacher led, children of this age for example unless they are more towards 7 years than 4 and have a lot of prior experience won't be able to go into groups to work on an improvisation. I would say that a good idea would be to use a story which takes you on a journey, bear hunt perhaps or where they meet different characters. Then lead the children through the differnent environments, narrating as you go encouraging them to think about how they are moving eg walking through mud it s really sticky, our feet are heavy etc etc. Concentration levels can be low so change activity often aand have a few things upo your sleeve in case something doesn't work out. Poems are also good to use, especially ones where they can incorporate actions or funny voices. If things descend into chaos then sleeping lions is always a good standby!