My Mom In Law is a vegan and she's very excited introducing this lifestyle to my kids. I think this is too soon for children they still need more nutrition. Please help how can i make her stop?

Do explain to her your concerns that you have with regards to nutrition and that you would like to feed your child all kinds of foods before letting them make the decision of becoming vegan at a later stage of their life when they are able to make their own decisions. I am sure since she only wishes well for your child and will be open to listen to you especially when the health and development of your child is concerned.