S.S.
I don't know about different journals, but I think it is a wonderful idea. Maybe another idea is little letters or loose leaf stories to give them on their birthdays (depending, I guess, on how they handle coming to grips and realizing the death as permanent.) For instance, on their tenth birthday, slide a little sheet of paper in with their birthday card or what not that starts with "When your mother was 10..." and relate a story that you remember. Or make several scrapbooks...one for each year of her childhood, for her children to "keep" or look at as they grow up.
With them being so little, what about "buying" a star and naming it after their mother (maybe one for the lost baby too)...for now it might help them process it a little by being able to point of to the sky and say "Mommy is up there, with the angels, watching over you. And anytime you need to talk to mommy, you can find her right there." As they get older they may appreciate the commemoration.