I have a bit of experience with autism and when I thought one of my previous dcb's had it, mom and I discussed it. He was the oldest in my care at the time and the farthest behind so mom saw the difference between him and the other children every day. He had multiple ear infections as a baby so during one of his doctor appt's she brought up how he was 3 and still wasn't speaking. They hired a speech therapist to come into their home a few times. She mentioned autism to me once and I jumped on it. I told her my experience with autism and some of the more typical symptoms. It put the idea in mom's head and I sent her a couple of articles to read over. Shortly after mom lost her job and ended up staying home with him until he's in school so I'm not sure what came about it. I am almost 100% sure he is autistic though.
Do the parents know you have experience with autism? I know because my dcb's mom knew that I did she trusted me and took what I said serious. I would explain to the parents your qualifications and go from there. If they decide to stay in denial there isn't much you can do but at least you know you did everything possible to try and help the child.