Inactive children have a much higher tendency to retreat into a fantasy world.

Literal

Inactive children [topic-は] fantasy world [into-に] shut-oneself-in tendency [subject-が] much higher.

不活発な = 'inactive/passive.' 空想の世界に閉じこもる = 'to shut oneself in a fantasy world.' 傾向がずっと高い = 'tendency is much higher.' ずっと intensifies the comparison.