How do I politely say on birthday party invites that presents are not necessary?

I am inviting my son's pre-school class over for his fifth birthday party and would like to tell the other parents that we would like for no presents to be brought. The party is for my son and step-son (who does not attend the same school, thus these parents don't know him). Also, in this economy, I know times are tight and my sons will receive enough from the family without the extra from school chums.