This infection, while benign in otherwise healthy individuals, can be quite dangerous in those whose immune systems are compromised such as those who are HIV infected or have cancer and are on chemotherapy. The risk of passing the infection on to the fetus is greatest if the mother has never been exposed to CMV before and then gets the infection during pregnancy. Knowing whether you have been exposed may be determined with a blood test, but this test is not a part of the routine screening during pregnancy. However, regardless of the actual outcome, the baby may secrete CMV in the urine for many months after birth. The infection is passed from one person to another by coming in contact with mucous membrane secretions.
Children pass it amongst each other primarily by coming in contact with each others saliva.
Adolescents may acquire it through kissing, drinking after each other, and by sexual contact. And adults who have never been exposed to CMV could get it from exposure to urine from infected children. Your question is a good one. This is due to the fact that this infection is so prevalent that special handling and exclusion of the children known to be infected would make no difference in transmission of the virus. In other words, there are so many children who excrete the virus that we don't know about that excluding the ones we do would be both futile and unfortunate for those identified children.