I'd not use chemicals on a baby that young. Hopefully the yard/area has been fogged prior to the party to cut down on mosquitoes to begin with. I'd also make sure baby is wearing a cap the entire time (nothing worse than a bite right on the skull!) Netting for when baby is in the stroller/carseat. Rest of the time, you are either going to need to be inside the house or just attend during the day when the mosquitoes are less likely to be around.
I'd also be a little hesitant to be passing around a less than one month old at a party, but that's just me. I know it doesn't bother some folks, but I grew up in an era where you limited baby's contact with a bunch of people at one time until they were a little older. I suppose it isn't any different than having baby go to daycare at 5 or 6 weeks, which many moms have to do.