I had this very difficult conversation with a parent before and it was not easy. I began it something like comparing it to "what if the child had signs of cancer" so for instance imagine if your child had even one sign of having cancer would you take them to see a doctor? Of course they would. What parent wouldn't? Why is Autism any different? Although the little boy in my care only had three signs I feel that it warranted me brining it up. Because what if he did have autism and I said nothing and the parents came back to me (their trusted provider/ teacher of their child) and said "but why didn't you say anything?"
I would have felt horrible. I brought it up just the one time and left it at that. At least I could go to bed at night knowing that I was a voice for the sweet little boy in my care. Sometimes parents of an only child don't really have any knowledge of ages or stages of any child and really rely on us. You are a huge part of that child's life.......and if you have a concern then you should voice it. Just the same as any health concern.
But that's just IMO :-)
You are so great to care and want to help. They are lucky parents to have you as a provider. If anything feel confident in that :-)