I agree that you are right to be concerned. I assume you're the Nanny, rather than the childminder? Do you take them to the GP, in which case you could discuss this with the doc then tell the parents that the doc had raised concerns? Or, possibly, with the class teacher? All this depends on how much contact you have with other professionals.
Perhaps one of your friends could be looking after a child with AS, and you do some reading because you want to support her - then leave books/articles where the parents can see them? A bit passive aggressive, but would make a good opener to a conversation - "I saw a fascinating article in yesterday's that said x out of 10 boys get to secondary school then find out they have AS"
It is unlikely they haven't noticed that there is something different about their son unless you have sole charge from breakfast to bedtime and the parents have very little contact? Good luck, and HTH