How do I tell my 3 year old her dad has died

I’m not really sure where to start. It was my daughters 3rd birthday yesterday and we all had a lovely time. However my husband (m/30) was involved in a car accident on the way to work. He stopped to help a woman who had hit a deer and then was hit by a lorry. The policeman said he died at the scene. I have no idea how to tell my daughter her life has changed completely and our son who is 6 months old will now never know the amazing man his father was but he’s too young to have to explain anything and can live in glorious ignorance. The three year old on the other hand will have questions. Please help. via /r/Parenting http://bit.ly/2Pz8LAq