Before the kids were born I read a book called "When Bad Things Happen to Good People." I can't vouch for the theology since I can't remember it and wasn't even a believer then, just someone hurting and searching for answers.
It helped, you may want to check it out.
I wish I had better answers. This week will most likely see some of these same kids questions addressed by the two youngest, and I'm sure DH will be hurting too.
I think part of this with kids is not over or under reacting to their grief. It's part of life, let them have it even if it hurts to see. You can't fix this one, but you can be there through it, even if you're grieving too.
A very specific thing happened a few months ago with Rachel. I was curling her hair before church and she started crying remembering how "mamma used to fix my hair before church too." I didn't pause, just kept doing it and asking little questions like how she did it, if she knew how much her mamma loved her, etc.. I'd turn her around as if I was trying to reach part of her hair in back if anyone started walking in so I could shoo the person away, letting her "air it out" without her realizing I was doing so.
Just being there. Even if she seems like she's mad or inconsolable and you're frustrated. Acknowledge. Don't fake an answer, sometimes there isn't one that we know, so it's okay to say "I wonder that myself too sometimes." Don't let her sit and dwell, if you see her doing that, give her a way to express it like drawing a picture of the loved one's favorite flower or writing a story dedicated to the memory of the loved one, etc.. If they sit there dwelling on it mentally for too long they get overwrought.
One thing I used to do with kids and haven't with these two yet is to find (or buy from a science homeschooling type place if you don't have them in your area) those little rocks that are just plain gray outside and are full of crystals inside. You can find huge ones, but if you buy them they're tiny. And tiny ones are easier. Buy a dozen, give her a hammer and chisel, and let her have at it. It releases anger and frustration and the rocks are really cool inside. Sparkly. Girls love 'em. It's also a lesson on what looks ugly on the outside is sometimes just a reflection on the polishing going on inside.