You're probably going to have to go down the MPV route here. With bigger kids, who only need booster cushions, you can get away with saloons and estates, but under fours need more space and, preferably, individual seats. 

 

Your best bets, then, are the Volkswagen Touran, Toyota Verso or a Citroen C4 Picasso. Something bigger and posher? Try an Opel Zafira Tourer or a Ford S-Max. Something expensive and classy? Volvo XC90.