Normally kids cannot concentrate for too long so the best thing is vary your activities and above all make them fun. Using music which involves some body movements will help. A simple example is "If you are happy and you know it CLAP YOUR HANDS." By singing and miming the actions you put in the song you will make such activities fun and educational.
Do not teach too much vocab each lesson. Make your lessons very focused. For ex. I am going to teach them 4 animal names and the sentence pattern "I like cats."
I have been working on a site that may help you. Check out a Free Unit with resources to try
I have a class of 18 3-4 year olds and don't know what to do with them. I tried "If youre happy and you know it..." today but they didnt show much interest. Everybody mentions a variety of actiuvities but I cannot think of even one activity that would be interesting to them
(except for the activity with a dice and flashcards which I found on this site, that one worked really well!)