Fortunately for me, my 3 year old little girl has only had two colds since birth - and they were very short.
But I was (and still am) unable to allow her to stay with anyone but myself, and I've no other children.
I have a cousin that has had her little ones in awesome little preschool programs and activities since they could walk and talk... and from that point on, they both seemed to always have little colds. One would get it from another child and then bring it home to their sibling. It seemed to me that they'd share colds on the average of at least once every couple of weeks.
So... although you're very thoughtful in keeping your children away from others while they're sick, not everyone shares in that quality. People send their children to school, preschool, ballet, etc. sick, all the time... unconcerned about the health of anyone else.
A few months ago I took my little girl to her preballet class and a mother was there 'hacking up her lungs'. As she coughed repeatedly, she reassured us that her child was ok not to catch what she had, because she'd already been through it. It didn't even enter her mind that MY child or even myself, the other children and mothers HADN'T gone through it, and that she was still capable of giving it to us.
So you see... some people are thoughtless (intentionally or not).
Good luck with your little ones!