Parents often worry about saying the wrong thing when talking to children about a death in the family and end up saying very little at all. Children generally cope better with clear, honest language than with vague explanations they have to piece together themselves.
Using direct words like died and dead, rather than euphemisms such as gone to sleep, helps avoid confusion and fear that can come from more abstract language, particularly with younger children.
It is also normal for children to ask the same questions repeatedly or to move quickly between grief and ordinary play. Neither means they are coping badly and both are a normal part of how children process loss.