I'm sorry for your losses :[
Hopefully your chicken wasn't taken by a hawk or an owl.
did you get your puppy from a pet shop?
If so then there is a big reason why that would happen.
Pet shop animals like puppies, kittens, birds etc. come from animal mills.
Puppy mills are horrible places. They are very cruel to their animals.
They will use young dogs till they can no longer breed then just shoot them in the head and throw them away.
Alot of animals die everyday because of the horrible conditions they are kept in.
They are normally shoved in cramped cages lying in there feces and some are never feed or watered properly.
They are never socialized like they are suppose to at that age which can lead to human or animal aggression over time.
Once most of the animals leave the mills, they are already sick or malnourished. 
If its not that then its some genetic defect from the generation being over bred which then causes behavior/mental problems in the animal.
So many people buy puppies and bring them home to there families and they end up dead in a matter of a few days to a week.
Its detrimental to the family/children and the animal. Its a very very sad thing :[.
Its better to just get a puppy from a shelter. If not a shelter then at least a reputable breeder. 
They are normally healthy and they are already looking for a home.
Anyway hope you don't take this wrong, just trying to give you some useful information on why your puppy may have died.
I'm so sorry for you family to have gone through this!