According to Life & Style, the doting royal parents want to make the occasion a memorable one.
'Kate is organising a lavish $100,000 birthday party for George in the gardens of Kensington Palace,' a royal insider told the publication.
'Kate doesn't always spoil her kids like this, but she wants George's 5th to be a day to remember!'
According to the palace insider, there will be 'lots of toy cars', including 'mini Ferraris for George and his friends to drive around the gardens in.'
'There will be hundreds of balloons and a huge Lego area where the kids can build. And Kate also plans to set up a little fairground and a bouncey house.'
Life & Style's insider adds that George's birthday will also include 15 entertainers - ranging from magicians to pizza makers.
Plus, Kate has already planned a show-stopping cake.
'Kate is going to surprise George,' the palace source spills. 'With a huge colourful, custom-made birthday cake.'