So glad you found us at Empowher. I have a few questions for you.
Has your 7-year-old had roseola before? While it mostly affects children under age 2, it is possible for anyone to get it.
Do you know what stage of roseola the 2-year-old was in? Kids normally have a high fever for 2 to 4 days. Then the fever goes away and the rash appears, which lasts another couple of days. They are most contagious during the fever period, until the rash appears. The problem with this is that most parents don't know the baby has roseola during the most contagious time.
Do you know if your son and the 2-year-old came in close contact? The virus is spread from secretions around the nose and mouth, like a cold or flu is spread.
I did not come up with any concrete information on whether a child could actually infect another child not by direct contact but through a carrier. I would say that if your son came in direct contact with the sick baby's mucus or saliva and then came directly home and also had direct contact with your baby, that it would be possible. Perhaps some moms out there will read this and have experiences they can tell us about.
Interestingly, one source I found said that roseola wasn't always contagious between siblings.
In addition to EmpowHer's page on roseola, here is the Mayo Clinic's page:
And here's a picture of a baby (poor thing) with the rash, in case that would help you know what to watch for:
How long ago was the contact between your 7-year-old and the sick 2-year-old? Have you noticed either of your children starting to have a fever?