Im going to go against the grain and say i would be annoyed and may even leave if i were at a party where there was an ill child even if just a cold. My DD for some reason easily gets ill and she always gets really ill even if just a cold so to me it would be an issue.
This is what I'm afraid of. I remember when DS was invited to a birthday party a couple of years ago. I was heavily pregnant, and DS had a cold so I rang up and said that we wouldn't be coming because he had a cold. The mum (who knew I was pregnant) said "Oh that's ok, most of the people coming have got colds anyway". I felt kinda hurt that she hadn't at least let me know, and had DS not been sick and we'd gone I'd have been walking into a party of a whole bunch of lurgies.