A good tip for young children is to give them a craft or art project to do. It doesn't have to be very complicated - just some colouring pages you can print from the internet, would do for toddlers and pre-schoolers.
You can either introduce the craft activity at the beginning of the party while you're waiting for every one to arrive (you can supply some pre-cut party crowns or hats and get the kids to decorate their own, or provide a large piece of paper to decorate and use as a placemat for their birthday tea).
Alternatively, if you have a room of excitable children, have them do the craft activity at the end of the party. In this way, they will calm down before being released into the community! Possible craft ideas are animal masks, icing fairy cakes or biscuits, designing and decorating their own party bag.