like L&B said there are often companies that will come to your house and host the parties, some will cater too, it depends on the $$ you are willing to spend. A number of them have themes and will bring dress ups and games with them.

A girlfriend in adelaide hired a rec centre and the kids got use the basketball court, indoor cricket ct, indoor soccer court and the beach volleyball room. The beach volley ball room they could use as a sandpit, all they had to bring was buckets/spades for the sandpit, food and drinks the rest of the equipment was there. If you were doing this I would consider combining them to save money and the more children the better when it comes to sports

or rope in some family and friends to help, teenage nieces and cousins or friends teenage daughters may be interested in earning some pocket money to play hostess to and arrange games etc with the kids and you could bake now and have most of the food reday in advance and frozen or buy food that only needs to come out the packet.

good luck