I have some experience with this,
The mother was actually the one that told me she was worried because her 2 .5 year old barely spoke. I later took a few classes with a speech therapist who agreed that there may very well be cause for concern. I gave the mother some paperwork that explained the normal milestones and reassured her that I was here to help her and her child. That my only concern here was to make sure her daughter thrived and was able to communicate her needs etc. I gave her the speech therapist`s contact info ( she really wanted to speak with the mother) But nothing came of it sadly. I did what I could .
I think they key here is to make sure the parents know you are coming from a place of love. You truly do care for their daughter and this is why you are intervening. Remember that no parent wants to hear their child is different and they usually shoot the messenger.
They may very well leave once you tell them but you have to think of the child first . Maybe they will look into it after you mention something and this girl can get the help she needs sooner.