How often do you have your daughter? Do your ex's parents babysit some while she's working? How much time is she with the JWs grandparents? Do you have Skype?
Try to instill fun, zest, quest for learning, interesting things for her - the opposite of JW learning. Encourage positive thinking, opposite of negative judgemental thinking of JWs. Her childhood time is her gift and does not belong to the grandparents to rob her simple childhood. A child of clear choices will choose the positive, fun things. I hope you will be able to be involved in her 5th birthday and Christmas, making it fun. If she parrots some comment that God doesn't like birthdays and Christmas, tell that is not true,,God thinks it's great. Teach her to 'celebrate' life. She will be drawn to your philosophy and spirit.
Children will draw away from negative, over-ruling grandparents, no matter what religion they are. If her grandparents in any way teach her contrary to your beliefs, and express disrespect to you, then you must limit their association some how.
I have grandchildren and step-grandchildren, having such a great time with much respect for their parents. Some are Christian, agnostic, Catholic and atheist. Everyone agrees with these three doctrines: love, joy and peace. Works all the way around.