First, I am so pleased that you're asking how to deliver this information. Thank you for being sensitive to another parent's feelings.
I agree with previous posters - Don't offer a diagnosis, have resources available, include positive observations. It may be obvious, but I would also not mention this kind of thing at drop-off or pick-up. I would schedule a conference with the parent (call it something simple like a mid-term progress conference) and give them the information there. Have concrete examples of your concerns, examples of work the child has done, or specific instances that have raised your antennae. If there's a handout that outlines expected behavior/accomplishments for that child's age, have that ready, too. Don't let the parent lead you into, "Well, what do YOU think it is?" It's tempting, but really - Let an MD diagnose. If you say something that is later not validated, they'll mistrust your expertise, and you may start to doubt yourself!
Again, thanks so much for posing this question. What a thoughtful thing to do.
Amy in NC
mom to Dixon, 2/14/2005
and Abigail, 4/7/2007