Thanks for the replies Jaycee and Tinks.

In the situation where I as host has been asked if a guest with a cold can come, does that change anything? If someone brings a child with a cold and I didn't know I can live in the bubble of ignorance But if I was asked in advance and I said yes as host is that bad?
Nah, I don't think it changes the situation. You can't be held accountable for that, what if the party was at a play cafe? As a parent you'd have to be aware that your child may and most likely will come into contact with a germy kid. I don't think this situation is any different.