Who does not like birthdays when it comes once in a year. Be it a party, be it food / drinks, be it games / activities- one enjoys the day like never before. Other than that it involves expenditure too which needs to be planned in a certain way. The ultimate aim is to make the people happy so that they shower their blessings of health / wealth / love unt you. And age is just a number, when a child grows, his maturity also grows.
1) A great cake: It is the most attractive part which will be kept in the center. Everyone would love to taste it when it gets cut and be divided. It can be made according to the theme for example you put red theme or pink theme. That'll be great when it will match the dresses. It could have prepared from the local bakery or home-made too. A better cake denotes another year added to person's life.
2) Food / menu: What is that, that will mouth-water the people's tastebuds. Punjabi or Chinese cuisine, continental or Indian, or Thai. Will there be soups, salads, seasonal cooked vegetables etc. Just ponder over it and let the party be memorable. Like marry the arrangements could have made and waiters could be there to serve it. A special chef could have hired that will suffice all the requirements and satisfy the gluttony. Wasted food can be discarded.
3) Gathering: Decide on how many people you want to call and take care of their sitting arrangements. They will all mingle up while you can play the music behind. Tambola also can be planned just to engage what you have called. People should not be more or less than the one which you can not handle. Choose carefully!
4) Introvert nature: Only those kids can talk to each other and be involved when they're asked to mingle with each other personalities. These are type A personalities who are confident enough to grasp and understand the human behavior. Push them to have a conversation with each other. They will talk about family, habits, experiences, their positions and negatives, strengths and weaknesses, insecurities etc. Let them have their own time to make interesting talks / gossips with each other.
5) Games: games like musical chairs, lemon on spoon, little games and dancing damsel can be played just to engage the children who you've called. Chits game can be played where everyone would be asked to share their best experiences of life till yet. Just make sure everything is clean and available. Washroom is clean and has good soap just to avoid germs.
So you did you best in organizing the party, just to end it you can give return gifts and let it be dolls, keychains, pens, laughing Buddha etc. Other option could be to give the party at a hill station like on mountains away from homes, changing of place / changing of weather could be a nice idea to spend time with children and their friends.