TusenFryd, an amusement park 10km south of the city, is enormously popular with kids from all over the Oslo region. The park offers carousels, a fantasy farm and an excellent wooden roller coaster, which creates zero gravity 12 times each circuit. You'll find it just off the E6. The TusenFryd bus (bus 546) departs from the corner of Fred Olsens gate and Prinsens gate roughly hourly between 10am and 4pm.
Prices are based on height, not actual age, ie children pay adult prices if they are over 120cm (and those under 95cm are free).