Explaining life

How do you explain death to a 3-year old? My parents had to put down our childhood dog on Sunday morning after she had an apparent stroke the night before. Emily turned 14 the Tuesday before.
We told Grace that Emily was sick and went to heaven to play with God. Grace knows that God is at church and she associated the two and now keeps asking to go to church to play with Em.
We'll just have to keep explaining as best we can.