Yeah, it stinks, but... we had to cancel my nine year-olds second birthday party because she had a nasty virus. Certainly harder when the kids are older and realize what's going on.
I too prefer to avoid indoor play spaces during flu season. I swear I got strep from Chuck E Cheese once. If it's a good friend, I'll send the kids and just make sure they wash hands well before we leave, and remind them going in to try not to touch their eyes or face. We've all had the flu shot this year, but that only goes so far

All that to say - for our winter birthday kid, we've steered toward smaller parties at home, or outdoor parties like ice skating.