Hate to hear...kids I think sometimes get closer to animals than anyone, even harder for them than us- I bet everyone in here recalls every pet they lost...but when younger, that pain is often the first/worst...
can never replace them, but sometimes its better than nothing...maybe in a while go pick out a new buddy...and tell her her dog wouldnt have wanted her to be sad, and woulda wanted her to share in caring for another lucky animal- most kids seem able to relate to that a little...cant replace them, but can still make new friends.
we had a stray cat that was the coolest animal Ive ever known, and I'm not a cat person at all, weve had several, all were a pain...but this one actually listened, never made a mess, never shed a hair, played- I mean played- with the kids/watched TV...was one of the smartest animals I ever saw, that includes some humans too

it got cancer at like a yr old, had to be put down, still miss that darn cat. a few months ago, the groomers had a stray looked just like it...son was that cats #1 fan, when he saw it when dropping off our dog for a trim, he just had to have it...we him-hawed around a few days, finally gave in...my son(and the rest of us) soon found animals are like people- they might look the same/act different, vice-versa...its not bad but nothing unusual for a cat, it does what it wants, when it wants, etc- he still loves it, carries it around half the time, but I dont know if that cat even knows he exists- but its his buddy, and he takes good care of it wether it appreciates it or not, so its not all bad

Did I mention I still miss that old cat?
