AFTER A COMPLETE WEATHER BRIEFING FORECASTING THUNDERSTORMS ALONG THE ROUTE, THE PILOT ENCOUNTERED AND ATTEMPTED TO FLY THROUGH THUNDERSTORM ACTIVITY. MANUALLY DIGITIZED RADAR AND NATIONAL TRACK ANALYSIS PROGRAM RECORDED RADAR DATA INDICATED PENETRATION OF A VERY STRONG (VIP LEVEL FOUR) WEATHER ECHO CONTAINING A THUNDERSTORM WITH VERY HEAVY RAIN SHOWERS, MODERATE TO SEVERE TURBULENCE, AND UP AND DOWN DRAFTS. RECORDED RADAR INDICATED THAT AIRPLANE CONTROL WAS LOST AT ITS CRUISING ALTITUDE OF 8,000 FEET. THE OUTBOARD SECTIONS OF BOTH WINGS, MOST OF THE HORIZONTAL STABILIZER, AND PIECES OF WINDSHIELD AND CABIN WINDOWS WERE FOUND IN A ONE HALF TO ONE MILE ARC WEST OF THE MAIN WRECKAGE. ALL FAILURES WERE OVERLOAD. THE TEN MONTH AND FOUR YEAR OLD CHILDREN IN THE REAR SEAT WERE RESTRAINED IN CHILD SAFETY SEATS AND SURVIVED. THE PILOT HAD BEEN GIVEN CLEARANCE TO DEVIATE AS NECESSARY FOR WEATHER AVOIDANCE.