As I interpret it, my eldest sits in the front with a booster when I'm not in the car. We only have 5 seats, so when I'm in the car, all 3 kids have to go in the back. Hence, the 2 littlies have a Britax seat each, and Will (7) has just a belt cos there isn't room for two Britax seats & a booster.
As far as the front goes, you can install a kid as long as it is not possible to put them in the back (we figure he is safer in the front with booster than in the back without).
Skoda recommend making sure any kids in front sit at least 300mm from the dash.
The only situation where you can't use front seat + bag is where you have rear-facing kiddie seat.