Pushy mother in law?

Question

Hi..my mother in law is driving me nuts. She keeps insisting that my 3 year old son needs to be in preschool. What is the big deal?? I am a SAHM with my 3 year old and 6 month old. I understand that sending him to preschool allows him to interact with other children his age...but if I handle that all myself...what is the big deal...I feel that it would just waste my money. We are a very active family and my son does interact with other children his age...at the park, play dates, play groups. We do this about 4 times a week...isn't that enough?

How can I tell her to bug off...but in a nice way?