I found one article that specifically mentions preemies being at risk for complications due to Serratia Marcescens. It is a rather alarming article though, and I would be careful to not jump to any conclusions from reading it as this child sounds otherwise very healthy. You have to be careful what you trust on the internet kwim?
I would however push for a confirmation from my physician of whether this is or isn't what you are dealing with and the exact risks involved, especially since the child is in group care. "Not really concerned" wouldn't be an answer I would be comfortable with after reading that article.
I found lots of info about how it is found in humid places like bathrooms etc, and how it is hard to kill. Nothing concrete about contagiousness, and the risk involved if you cannot kill it. That is what is driving me nuts.
Edited to add the article link:
http://deadlymicrobes.com/serratia-marcescens-symptoms