Agree with Wish - It should be seen as a positive thing that the SENCo wants to meet with you. It's fine for you to go along and support her, your daughter doesn't have to go alone, but it shouldn't be anything to be intimidated about, it will be talking with you about any areas that your grandson is struggling with, and then together putting a plan in place as to how you can all work together to help him.
In our area, the first SaLT (Speech and Language Therapy) appointment can be 10 - 12 months after referral, so it would be good to put some strategies in place, to support his language development now, to see if he can make improvements in the meantime. SaLTs assess a child's understanding of language, as well as their articulation - sometimes people think they only deal with the way a child pronounces words, but that is a small part of their work. It will be good for the staff, his family, and anyone else who works with him to be clear about how much of what is said to him is being understood by him, then, once again, strategies put in place to ensure he is understanding what is being asked of him.
A request to get hearing assessed is again, a really positive thing, as clearly it is a good thing to rule out any impairment in his hearing.
Presumably if he has been sitting on the thinking chair, then he has done something that has been considered inappropriate behaviour. You might want to ask about those things - what has he been doing, etc., and help him to understand the "rules" of the Nursery and help him understand (if he doesn't already) why he had to sit there, and what he can do/not do to prevent him from having to sit there again.