The finality of death is hard for a child to comprehend. Your daughter is tasting the edge of that when she misses her Maddi. I would sit with her and help her fully experience her feelings. Empathize, let her know you feel sad, too.
Missing Maddi is both sad and sweet, so help her notice the sweetness. Tell her Maddi would want her to to happy they had time together, and that someday the sadness will get smaller.
We use this approach with my grandson, who lost a beloved pet at two and a beloved great-grandma at three. He still remembers both with tenderness, and has learned to accept that the sweetness of that love is still there, right along with the sadness of missing those dear ones. It's all part of the fullness of life.
I love the thought that the heights of our joy will match the depths of our sorrow. We can't experience one without the other. It's fine if your children see you model a willingness to feel grief and loss, and then acknowledge that our loved ones want us to feel happy again as soon as we're ready.
It may also help to introduce the idea of taking turns. Every person, animal, and plant has a turn at living its life. When their turn is finished, they die so other people, animals, and plants can have their turn. For most of us, our turn is over when we are so old and sick that we aren't enjoying being alive any more. I also avoid equating death with sleep, because some children become afraid to close their eyes at night.