I was given this idea by my daughter (year 5!) who was enthusiastic about her teacher doing this...

Decorate an empty biscuit tin with sugar paper and put candles on the top to make it look like a birthday cake. Each time it is a child's birthday, present the 'cake' to the birthday child singing happy birthday with the class.

I have found that children enjoy the attention of blowing out the candles (or just pretending to).

The tin can also be filled with cheap lollies to be given to the birthday child.