Name debate. In law

When deciding on a name for my son i was debating on Liam James or Liam Charles. James is my fathers name and Charles is my husband and father in laws middle name. I was worried when I was chosing Charles that people would want to call him Charlie and I didn't want them to. The reason I went ahead and chose Charles is because my husbands other son is Noah James.

Well now that my son has been born(in January) my mother in law and some of hubbys family, insists on calling him Charlie. I have said something to her but she didnt seem to listen. How can i get her to not call him charlie?

Her excuse is hubbys niece(12) cant say Liam, but the 5 year old can. I would understand is she has some kind of issue keeping her for saying it so she called him charles but no I think they just do it because I dont want them to. I believe my husband has said something but he hates conflict and tries to avoid it so it normally makes me look like the bad person.