Setting the scene:
The key component to parenthood seems to be children, so decorate with all the trappings that come with kids: everything from cribs and strollers to tricycles and Big Wheels, pompoms and skateboards. Children tend to lay waste to quality decor, so add a few highchairs replete with bowls of Cheerios, place some Tonka toys where your feet should be, and have crayons and coloring books available to all. Add balloons, diapers, dolls, homework and applications for driver permits for a final touch. Invitations should be made of construction paper and feature crayon art and fonts. Party favors can include any of the things you've decorated with - seriously, just get it out of the house as soon as possible.
It really doesn't matter what you wear to a Parenthood party, although fake pregnant bellies would be funny. Just be sure everyone gets a Tide to Go stick and a bib. Kids are messy.
On the menu:
It's kid-tastic cuisine! Go with kid favorites such as macaroni and cheese, chicken fingers, hot dogs, pizza, peanut butter and jelly sandwiches, Happy Meals, jars of Gerber baby food, and Ziploc bags full of Cheerios. All beverages should be served in sippy cups unless they're juice boxes.
On the hi-fi:
(You're) Having My Baby by Paul Anka, There Goes My Life by Kenny Chesney, Daughters by John Mayer, Sweet Child O' Mine by Guns N' Roses.
Nothing says "We're parents" like having a huge inflatable bouncy castle in your front yard surrounded by a few rented ponies. Give a shout to Unique Inflatables, and either order a fairy-tale castle or have a custom bouncy made just for you!