DD's 4th birthday party is on Sunday - when 'showers' are forecast. 
Judging by how my DC don't really mind being out in the rain, WIBU to still hold some of her party outside too? I've planned games etc that just won't work inside.
Perhaps if I asked parents to bring the DC's raincoat...?!
Most of the kids are her age, a few a bit older, a few a bit younger.