More and more doctors are doing detailed screenings at the 18 month check up and they may very well identify some of the same issues you are sensing. If you really think there are issues when the parents say they are going to the 18 month appointment that might be the time to bring up a couple of your concerns and ask if they will mention it to the doctor and get some feedback on how you can help the child over come the problems. A knowledgeable doctor will be able to take your list of "symptoms" and get that something is off in the day to day of the child even if it isn't showing up at the appointment.

Until the parent is willing to admit there are issues the best you can do is take what you have identified and work around them or with them as the case may be. Just be ready to confirm to the parent if they bring it up that you have been having concerns too.