Hi, I could probably give a really long analitical answer from different points of view but due to my limited time I would suggest you to try to do this,

Take or print a list of developmental mile stones for that age, follow the child and check ( make short notes, etc )

When your done, just give a copy to the parents and tell them to ask to their pediatrician (if all those signs are normal or not in child developent), that way you did your job, and another professional will give or support also your opinion. You did what you could and now it is up to them.