it is a common practice to include gratuity charge for a large party greater than 4 but 20% sounds a little too much. 
if you did not order, you have the right to cancel and refuse to pay. stand up for what is right..dont hesitate that people around will feel that you are cheap. your kid (assuming he is old enough) could have learned a valuable lesson had you stood up for the right cause by refusing to pay!

come on man, when are we nepalese people going to stand up and stop being shy??