The guest list - work closely with the birthday girl/boy on this, firstly deciding on the number that are to come, and then whom. Some parties seem to have all the children of the class invited, and if you are happy to do this, great. But if you prefer something smaller, both for financial reasons and for more of an intimate gathering, set the number low and perhaps look at emailing or posting the invitations so that classmates not invited won't get offended. A good rule of thumb is to invite a friend for each year of their age, eg a 2 year old would have 2 friends come to her/his party; a 6 year old would have 6 friends come to his/her party.