S.,
I am so sorry your mom passed away so early in your life and in the life of your children. May God bless and comfort you as you continue to grieve.
I think it is good to be honest with children about death (after all, it's inevitable) and to allow them to grieve, but to keep it wrapped in a parameter of being a little brisk and practical. In other words, acknowledge the feelings and be comforting, but in a very matter-of-fact way..."Yes, I know you are still sad about Gamma, but we can be so happy that she isn't sick anymore and she is enjoying being with God." Or, "I really miss Gamma, too, but it makes me feel good to know she doesn't feel bad anymore, and one day, we will all be togther again. For now, she would want us to think about her but to also be happy."
It may help to begin to shield her from any continuing grief you experience, just to give her a sense of boundaries about it (to demonstrate that it is okay to move on and not always feel the grief, although you actually are. She needs to have a more concrete ending).
And again, I think it is great that you have grieved honestly and openly with her. But now, whenever you feel overcome, you might want to express it in private, unless it is something you can still be matter-of-fact about: "I'm really missing Gamma right now, but I know it is okay, and I'll feel fine again soon."
I know your husband feels protective of you, but I hope he can realize that your little girl is just trying to understand the dynamics of grieving...and it's not unusual for kids to try to figure out just how much mileage they can get out of any emotion! - that is just how they learn. So I hope he can be patient and matter-of-fact with her, too, and just remember to acknowledge the loss and the pain, but to also move her back into the moment: We miss Gamma but we're happy she isn't suffering anymore. Now let's think of something very happy that will help us go to sleep tonight. What is your favorite thing to do on the playground? Guide her back into a different focus. I think kids confuse their grief over a death with their "just as real" sadness at not getting their way! It's just a lot of emotion to sort through for a little one.
S., I hope this helps, and again, I am so sorry you don't have your mom with you anymore. I lost both parents within 14 months of each other, both unexpectedly, so I know it is a very painful time. But it is also a great opportunity to teach our children the reality of death, and as believers, the hope and promise of seeing them again one day. May you feel a special measure of comfort and peace today!