Complex sentence: Over time, everything in the world becomes childproof after fear of repeats of Phineas and Ferb's dangerous activities and Doofenshmirtz gains the upper hand, effectively becoming emperor of the world.
Split into simpler sentences: Over time, everything in the world becomes childproof for fear of a repeat of Phineas and Ferb's dangerous activities. Doofenshmirtz eventually gains the upper hand, effectively becoming emperor of the world.