ok, let me see... lots of ideas so i'll just throw them down and you can pick out what you like:
1. theremin could fit into work being done on pitch or linking music and ICT
2. start by asking the children how they think you make the theremin work - they will suggest twiddling the knobs etc. Then ask a child to walk towards it - ask them to describe what is happening. would it work with just your head moving? would it work if we moved a stick instead of our hand?
4. explain how it works - if anyone could write a child friendly explanation of how a theremin works i would love it!! i don't really get it myself.
5. demonstrate making the theremin get higher and lower. Ask the children to make a physical movement as you change pitch e.g. raising/lowering their arms, gradually standing up/sitting down
6. demonstrate the volume aerial. One child could work the pitch aerial, another could work the volume aerial
7. Play them a piece they will recognise - over the rainbow, a nursery rhyme. - make sure they give you a big clap!!
8. There is a virtual theremin availble online and some lesson ideas at http://schools.becta.org.uk/index.php?section=tl&catcode=as_cu_pr_sub_10&rid=9332
which the music teacher may want to use to follow up your visit. I can't say whether it's any good as i seem to have a problem with getting the thermein to work. If you manage it, let me know what it's like.
9. some schools may have access to a soundbeam (a little similar to a theremin, used with special needs children) that the children could use after your visit.
hope that helps - if you go into school let me know how it goes - and if you're in the southampton/portsmouth area you can visit my schools any time! We could do duets!!
h