I'm sure the parents know something of your own children? I would tell the parent that you aren't sure whether to say anything because you are not sure if you're over concerned (because of your own experience) and explain the things you see, or what could possibly be signs, and why they concern you. Take your cues from their reaction and go from there. (The first time, if it is not something that has already crossed their minds, they will most likely just listen, or deny/brush off (shock) then think and observe.) Then if they decide or see for themselves that there is something to be concerned about, be there as a resource for them.
Early intervention is wonderful, if it is warranted. You might be their nanny because of your own experience and knowledge that could help them. So don't brush it off just because you could be hyper-aware. At the very least, if I were the parent, I would want to be aware what are or are not signs of autism and what to do about it, how to find out for sure.
Put yourself in their shoes, do you think they would appreciate knowing, or are they already observant and diligent parents who check out everything that might need checking? Even, and especially, if they leave it up to you, then don't ignore it.
For most parents, I think generally it is not the "problem" itself that is overwhelming - it is knowing where to go, who to ask, how to help, how to deal with it, what to do, etc. And here you are, a personally experienced nanny! Go slow, present & make them aware (tell how to get it checked out) then just be there and little by little as they ask/are ready for it, help them along this path.