How to combine a first and third yr old elmo bday party?

Hey im trying to Plan my sons bday party im going to do a first and third year old party my theme is elmo i want to get some ideas of how to combine this party together i want to do one cake and 1 invitation for both of them one is turning 1 and the other is turning 3 yrs old.. any ideas or suggestions will be greatly appreciated.... thank you.