Mia,
I can relate to you and your child and husband. My in-laws have not accepted that I have married their son. They like me before but it was like "I took their baby boy away". It has gotten to the point that they always put the other grandchildren ahead of my 3 children. My husband and myself has always said that we would not put our children in such a unhealthy environment. Just becuase they don't like me I don't want them to talk badly to my children about myself. So my husband being the wonderful man that he is and he stands up for what he feels right and believes in. He told his parents that if they could not accept me as his wife that they would have to deal with it and move on. He does not tolerate them talking badly about me. He has told them that their way of living is not acceptable for our children. They have different rules for each grandchild.
You have to remember that you child will understand in the long run. My 8 year old is already seeing it. I do not talk poorly about her grandparents. She just hears what they have to say on the phone or when they stop by the house when I am at work. This is when My husband has to ask them to leave.
My 5 year old understand also. She always tells me that she wants to be in a home that she can feel the "love" around her. She says she can not feel the love when she is with His parents.
Children at young ages are a lot smarter than what we give the credit for. Do what you feel is best for your child. I have and I have no regrets. In the long run the way we see it is that the grandparents are going to be the one hurting because they missed out on the best years of our childrens lives. Just because they wanted to play high-school games.
Stand up for what you feel and believe is right for your own family.