You are creating a birthday party for your child. Unless it is a surprise party, you should always involve your child in the party planning and preparation stages. This gives them ownership of the party and allows them to get excited making it that much more of an memorable event. The anticipation for the grand day alone will keep them pumped in helping make sure everything that needs done is completed.
Of course, if it is a toddler party, although their input is great to get, their preparation for the event should be made to be age appropriate. Allow your child input on theme, location, menu, games, guest list, and decorations. All ages of children can help fill party favor bags and older children can help fill out invitations, and decorate.