This is something we're dealing with now ourselves. My middle DD's, who is 8, her ferret (Penny) is dying from cancer, and its just a matter of time now, this will be their first dealing with death. I've been going over this in my mind. This is what I've come up with. I'm not overly religious, but I'm going with God on this one.
Having a pet is a special gift from God, and he shared Penny with us for as long as he could. Sometimes God shares a pet that is extra special, one that might not be perfect, one that may be sick and needs the extra, extra love that a special family like ours can give. God knows that the best house for Penny was ours. Penny is some place now where she's not sick anymore, we don't have to give her the medicine that she hated, she's running and playing now without pain. I don't know about you, but I wouldn't trade my time with Penny for anything, and even though Pennys gone now, I'm so glad we could love her just like she needed, and she loved us back. Now here is the hard part that you need to try and understand, animals don't live long lives like people do, and if you open your heart and take a pet into your home you will have to go through the day when that pet goes back to God. Its normal to cry and miss Penny, but also try and remember how much fun you had and now you have a little ferret angel watching over you.
I hope that helps, that pretty much what I'm planning on saying. My heart goes out to you and your family