You should ask Reverhart or Reality One for advice around here...they helped me in the past on a similar setting...
Thanks Luis.
As Rick said, this is a tough age to perform for. You have to establish yourself very quickly or they will challenge you. Start by dressing the part. Don't look like you are going to a kids birthday party, look like you are going out to a nicer restaurant with some friends.
Start with a strong effect like the invisible deck or Brainwave deck. Toss out the deck to the audience and have the first person pick a color and toss the deck to someone else, have the second person pick the suite and then toss the deck, have the third person pick number card or face card and then toss the deck and have the last person name the value of the card and toss the deck to you. Ask them if they believe in magic; if they believe in coincidence; if they believe in the power to control people's minds. Ask them if they would believe that there is once card turned over in the deck.... You've got their attention, you've got them involved, you've got them to voice their skepticism about magic and you've got them AMAZED. You've set the stage for the rest of your performance.
Just off the top of my head, here are some other effects that would work (Rick's recommendations are great and I won't repeat any of them): Mark Wilson's Tic Tac Toe, Miraskill (Bannon has a great version called View to A Skill), Vanishing Bandana, Sankey's In a Flash, Dan Haas' Flow (put bottle over spectator's head), Out of this World (a great effect for setting up for OOTW is in Card College Light), Daniel Garcia's Torn, Paul Harris and Wayne Houchin's Invisible Palm Aces / Shuffling Lesson from True Astonishment.
Notice that most of the card effects have the audience as the hero. The magic happens in their hands. Most of the other effects require a spectator to assist you. The only exception on Vanishing Bandana which I would use as the closer (so do you know how magicians learn tricks... mail order!)
Rick nailed the advice. Kick up the magic to another level, don't make your volunteers look bad and give the kid who volunteers the chance to look good. The only other advice I have is use good patter. Don't use the say-do-see presentation where you say what is going to happen, do it and then tell the audience to see what happened. Engage the audience. Make it interesting. Have fun!