I am so late to this that my input may not be seen, but here goes anyway. My DD is an August baby. I knew she would one of the youngest in her school year and I was worried about her slow speech development. One sound she couldn't say was S . Unfortunate as she is called Sarah. I decided that speech therapy was needed but knew that the chances of her seeing one were limited. So I approached it from the "I am worried that she may be deaf" angle. As a result she had umpteen hearing appointments, including the local university. Then got to see a speech therapist. I think it helped and was quite fun. For me the highlight was a series of pictures to teach them certain sounds. Unfortunately for them the picture of a car was a volvo estate just like ours. No way would our DD say "car", only "volvo" !! Thirty years on, my DD has a stack of high A levels and a 1st class honours degree as well as a high level job in her chosen career as a speech therapist!.
Make sure there is no physical cause for her speech such as deafness or tongue tied. Then just talk a lot with her, sing songs, learn nursery rhymes etc. When she talks, repeat what she says in a relaxed casual way as part of your conversation. Not as a " this is how to say it" way. Let time take its course. And perhaps start the ball rolling for Speech therapy. But try not to be over anxious. The worst possible thing would be for your GD to feel that her speech is "wrong" and for her to stop talking all together.