Depending on the illness I probably wouldn't make a point of telling them beforehand. In cases of bad sickness bugs then yes, I would but otherwise, no. There's plenty of bugs whilst walking in the streets unless they're breathing face to face it's highly unlikely the bug would be that contagious.
Let's be fair, clients only tell us they're full of cold when they're sat within 10 inches of us having nails done :o
I have been full of cold all week and carried on working whilst I would've loved to have been tucked up in bed, I think clients would think I'm unreliable if I cancelled over mine or my son's sniffles... unless your child is going to need more attention than normal cleaning up sick etc I would imagine most people would be fine with it.