Dear Ajay
Before you start you need to find out what they already know - so you don't go in and teach that.
Pick a new grammatical structure to teach them, and use familiar vocabulary - that way you won't be teaching them all new stuff in a short demo. My approach would be to have the children using the new structure by the end of the demo - to show that you get results and the pupils enjoy it.
If you have beginners then I would teach a handful of new words or very short sentences, such as "I like butter". If the goal is to have the students using the language by the time the demo is up then you don't want to be over ambitious.
I would do this using games - now my experience is with
beginners to intermediates, and mostly
children. That's no doubt why I find games to be the best method of teaching - and especially in terms of interacting well with the class. (The pupils love you.)
Referring to the above post which asks what the emphasis should be - whether speaking or writing, you can use games for either - but they are particularly effective for listening and speaking.
Also, if you are going in with games and activities, then have a few spare ones under your belt so you can drop what you are doing if you see it isn't working.
I would be pleased to put up a suitable game for you to try here if you let me know the size of your class and the age, and the level.
Let me say now that if you are teaching advanced adults then that is not really my area of expertise.
Looking forward to hearing from you,
Kind regards
Shelley
Free games and ideas to make teaching children more fun. Receive free games here:
www.teachingenglishgames.com