Explain it the same way I explained it to my daughter when she was 5 years old and lost her great-grandmother, whether you are a spiritual/religious person or not. What matters are NOT your personal beliefs but alleviating an emotional situation for a child.
I told my daughter that her great-grandmother had gone to Heaven because God needed some extra angels to help take care of people in Heaven, and that her great-grandmother was now an angel and was watching over her every day.
My step-daughter's children are 4 and 2 and lost their daddy right after the youngest was born...she explained it to the oldest by saying that God needed their daddy to help take care of little kids in Heaven. Every year on the daddy's birthday, my step-daughter will let her kids write messages on helium balloons (and we do it, too, to give the idea more oomph in the kids' minds) to their daddy and then we go out into the yard and let the balloons go. The kids think the balloons go all the way to Heaven and get read by their dad. It makes them feel better, and that's what matters.