How do i tell my kids there mum is dead?

hi this evening my wife was travelling home from a day out shopping when she came across the scene of an accident being the good samaritan that she was she got out to help and as she was ccrossing the motorway to help the injured people a car struck her with such impact that she died instantly she was only 41 years old and had everything to live for .when i got the news i rushed straight down to the scene where they were about to take my beautiful wife away on a stretcher it hasnt hit me yet that shes actually gone when i got home my four kids who are 16,15,8 and 2 asked me where there mum was all i could say wasshes gone out with her friends to a party and shes staying away tonight im now feeling so guilty about telling them that but i cant bear to tell them that their lovely mum is gone .please help me and give me a few suggestions that i could say to the kids in the morning i want to be completely truthful with them