These kiddos' disabilities are invisible, but not unnoticeable if you're aware of behaviors exhibited by the child and/or the seemingly odd parent/caregiver support offered to the child. What might appear to be "helicopter parenting" or "poor discipline" is simply our way of teaching and protecting our children in environments which are not created with their needs and abilities in mind. We discipline differently - according to the child's stage of development and his or her ability to comprehend what is appropriate. In our world, development and abilities vary and "age appropriate" means Jack.
There may be a 6yo who steals your snacks at the park, takes your toddler's sippy cup, won't move out of your child's way when asked, makes odd vocalizations yet doesn't speak, chews on things/licks things, wears diapers, etc. Or maybe a 16yo behaves in a similar manner. Always speak to these kiddos as you'd want someone to speak to yours. Parents and caregivers will assist, but these kids like to be acknowledged (even when it appears that they are not even aware of you) and they are learning how to navigate their way in social situations.
If in doubt, ask about our child. Autism is not a dirty word. Autism parents know that our children do not fit in, and we are aware that our kiddos are different than yours in many ways. Mentioning Autism or DD (Developmental Delay) will not shock us and will most likely assist us feeling comfortable enough to advocate for our children through discussion and Q&A.
Also, I must mention the parents who are not yet accepting of their child's Otherness and the Otherness of their family. I was once that mother. I would respond with snark or go on the defensive when someone mentioned my child's odd behaviors. I would cry at the drop of a hat. I was a terrible advocate and I had no support within the Autism community as I was in denial and scared shitless. Be patient with these parents. They'll find their way eventually, but they have been thrown into a world where they never thought they would find themselves. They were promised and sold on this idea of "normal child" - as were we all - and they are scared and probably a bit sad at missing out on what they believe the majority are experiencing.
As always, check out daamit.org for information about Autism and resources for those who are autistic and/or those who parent autistic children. Suggestions about the page are also welcomed.