You can defuse her argument easily by agreeing with her. She's only five years old, so her reasoning skills are not very sophisticated.
Tell her, "God knows everything, right? ...and God is all-powerful, right? so nobody can resist God, can they? So everybody is doing what God wants them to do. If god want you to go to the Kingdom Hall, you should go. but if God wants you to go to the zoo where you can admire his creation, then you should do that too. god likes it when we admire the things he's done for us. Why don't we go to the zoo on sunday and admire God's creation?"
Let her grandparents and her Mom tell her that Jehovah is not all powerful and that jehovah CAN be resisted. then you can ask why her Mom thinks Jehovah is so weak.
Confusion can be the first step toward truth. Confusion is cognitive dissonance: the conflict between two opposite ideas.
Later you can talk to her about being happy. Is Jehovah a happy God? Does he want us to be happy? How can you tell when people are happy? What if someone says they are happy but they really are not happy - can you tell they aren't really happy? Do the people in the Kingdom Hall look happy, or do they look sad? Is Mommy happy or is Mommy angry? If Mommy did what Jehovah wanted, wouldn't she be happy?
Truth is simple.
Keep your arguments simple. Let her Mom and her grandparents beat their brains out trying to explain how complicated "the truth" is.