Your party focus will be the game of croquet. As there are only six players for croquet, it may be necessary to have other activities for those who are not playing.
A tea cup "tea" bag toss or corn hole is a great side game. To make your own you will need:
A Mad Hatters tea party theme makes a very relaxed party as there is just tea and not really a true meal served during the party.
The tea and cookie table should be set near the croquet court so that the guests are free to eat and sip their tea in between turns or while they are watching.
Unwrap gifts and sing happy birthday after everyone has played a game of croquet. Maybe sing happy un-birthday to everyone else afterwards.