One of the main pieces of advice to follow to prevent the child seat getting scorched by the sun is to park the vehicle in an underground or covered car park, or in a shaded area. Direct sunlight should be avoided whenever possible. If you park in the street, it is possible to fit sun shades, both on the side windows and the windscreen to ensure that the least possible heat gets in. It is also advisable to open up the car a few minutes before getting into it and to put the air-conditioning or climate control on to adjust the temperature of the interior.
Also recommended is the use of seat covers specifically made for the summer and even to cover up the seat while the car is parked. There are currently specially manufactured seat covers available on the market (sun shade covers and anti-UVA covers).