A source told Closer magazine: ''Victoria's spending around £50,000. The garden will be filled with big pink cuddly toys, pony rides, a bouncy castle, balloons and tons of cupcakes. It'll be totally pink.
''It sounds odd, but she's even hiring special dyed-pink rabbits.
''They haven't had a house party since they got their Beverly Hills mansion totally renovated so they wanted a big Celebration to show that off too.''
The party was initially planned to be an intimate occasion but the plans rapidly escalated.
A source close to the couple - who also have sons Brooklyn, Romeo and Cruz - said: ''The party will be before Harper's actual birthday on 10 July, as on the day they'll be in the UK where they'll have a small celebration.
''It started off so simple with Victoria thinking it'd be cute to throw a party with all her friends. But with the excuse of a housewarming and Independence Day on July 4 coming up, it escalated into a proper showbiz bash.
''The guests will be all the Hollywood parents and kids - Victoria often has playdates with other celeb mums such as Selma Blair and Nicole Richie. And obviously, her best friend, Harper's godmother Eva Longoria, will be there. Harper will get some amazing gifts.''