First, you can tint your car windows.
Clear glass magnifies the sun and intensifies the heat.
If that's too expensive, buy manual sunshades. They come in cheap cardboard or reflective material. You can also buy shades for the back side windows where your kids are. To avoid hot steering wheels, put a towel over the wheel to absorb sun while your car sits. Most cars now come with a clearcoat finish, which tricks owners into thinking they don't need wax.