I don?t think you can say anything to the mother as such, chances are that she already knows there?s something wrong.
About a year ago I was in the park with my ds (then 3), when a woman came in with a little girl. She put the little girl on the swing and when I asked how old she was the mum replied ?she?s three?. It turned out she was about three days younger than my ds, but she didn?t talk, not at all, not one word. I didn?t ask the mum if she could/couldn?t talk, tbh considering I didn?t know the woman I didn?t consider it any of my business really, and wasn?t sure she would want people making assumptions about her dd, but as time went by and my ds tried to play with her, and they did interact, except the little girl didn?t say one word, she explained to me that the hv/other experts thought that she was possibly profoundly deaf, as she had reached all her other milestones, but didn?t talk/respond to sound/commands etc. She said that her gp/hv had been pushing for her to go for hearing tests, but that she and her dh were very reluctant to let her have a hearing test. I was somewhat dumfounded by her level of denial, and perhaps I was out of order, but I asked why she didn?t want her dd?s hearing tested, and she replied ?because I don?t want our lives to change?. I think what it came down to was that she, in her eyes, had a perfect dd, who had nothing wrong with her, even though she knew in her heart that she did have something wrong with her. And while she didn?t have any kind of diagnosis she could still keep believing that her dd was ?perfect?, but as soon as she took her dd for tests, she knew that that would probably result in a diagnosis, and once that happened she would have to face the fact that there was a problem, and in her mind, that would change their lives.
I never saw her again, but I?ve often wondered what happened to her and whether her dd ever had the tests/a diagnosis. She seemed so alone and unhappy (the woman that is) that I can?t help thinking that being there knowing your child has something wrong with it but not being able to admit it must be a very lonely place to be.