Chances are, if you've noticed peculiar behavior in their child, then they have, too. Sometimes, however, parents may become blind to any possible disability their children have because of the prejudice toward disabilities in our society. If you believe this is the case, then a direct confrontation would only prove counterproductive to your cause, because it would force the child's parents to confront a problem they obviously don't want to address. Perhaps a less direct approach should be taken, such as mentioning some of the child's odd behavior to the parents, and leaving them to draw their own conclusions and decide the best course of action for themselves. Your opinion, while quite possibly justified, can inspire spite from the parents because of your age and status in comparison to theirs.
Chances are the mother already knows if there is a problem.
Also, chances are there may not be a problem. I would advise you not to say anything. One of the things that has always made me furious is an early childhood education person wanting to tell me how to raise my child, especially when that person has no children. I learned this when my daughter was a baby. Nothing in any book ever pertained to my daughter.