The Qick Guide to successful children's parties
The Quick version of The Complete Children's Party Survival Guide

Presents

Do not let the birthday child open the presents at the start of the party.
 
     
 
Make sure that the birthday child understands that they will not be allowed to open the presents.
     
As the children arrive place all presents to one side, preferably in a large box or similar.
 
     
 
Ensure that the birthday child thanks the guests properly for the presents and welcomes them to the party.
     
Leave the present opening until after the party or better still...
 
     
 
...Organise a fun, present opening game as part of the party.

For more detailed information on the above points see Chapter 5 of:
IntroInvitations