I think it is wrong. An ill child should be at home, preferably quietly lying on the sofa and preferably still, in bed. (just my opinion I know!) I don't think it is relevant that the child might have been desperately keen to go to the party, and I also don't think it is relevant that everyone at the party might already have been exposed to the bug at school/playgroup.
It is selfish to spread the bug around more than is necessary, and it could even be harmful to the child who is already ill.