The pilot reported that he taxied the two-seat Cessna 150 airplane from the hangar onto the privately owned 2,000-foot grass airstrip and departed to the north. A witness stated that he observed the airplane takeoff, but it was slower than usual. The witness stated that the airplane usually gained altitude quickly after takeoff, but it seemed to nose-up and began "plowing forward" without gaining altitude. The airplane banked hard left but it hit some trees. The airplane nosed over when it impacted the ground. The pilot, his son, and his 18-month-old grandson received minor injuries. The pilot reported that the engine was operating properly. The inspection of the airplane revealed no pre-existing anomalies to the airframe or engine. The pilot indicated that there were 100-foot trees north of the departure end of the runway, but there were no obstacles to the south. The winds were from the east at about 9 knots. The local temperature was about 90 degrees Fahrenheit, which produced a density altitude of about 2,914 feet. The pilot reported that the accident could have been prevented by a "more thorough assessment of density altitude and choice of runway heading for departure."