I am sorry for you to hear it died. Other than poisoning, it may have sustained internal injuries before you got you got to it. Or given it was still associating with its mother it may also not have been fully weaned. Who knows?
I am not good at IDs from pictures for small mammals - so many look so similar. I can, however, tell you a couple of things to look for in the future. Rodents (Family Muridae) have two upper and two lower incisors. There is then (in most cases) a substantial gap in the teeth, before you get to the grinding premolars and molars. They lack any canine teeth. Almost all of them are totally herbivorous. The small marsupials (Family Dasyuridae) are carnivores. They have four pairs of pointed upper incisors and three pairs of lower pointed lower incisors, then upper and lower canine teeth, followed by a cutting ridge of premolars and molars. They tend to bite with the front teeth to subdue or kill their prey and then tear off bits using the side teeth when eating.
As a generalisation, the carnivorous dasyurid marsupials tend to have a longer, more pointed snout than the herbivorous murid rodents.
It certainly looked like a rodent that you had. An exact location of where it was collected would assist in narrowing down the possible species.
Sorry I cannot be more helpful than that.
Blue
Hopefully someone will be able to identify him now 
