Hi Daniella,
Disneyland in California and
Walt Disney World Resort in Orlando are two completely different experiences. Each has its own feeling and charm.
Disneyland in California consists of two parks - Disneyland and Disney California Adventure Park. Each park is about a solid day of touring and a third day allows you to visit your favorite attractions again or go at a slower pace. The two parks are very laid back and my family never feels rushed when touring these parks.
The Walt Disney World Resorts in Orlando consists of four main parks - Magic Kingdom, Epcot, Disney's Hollywood Studios and Disney's Animal Kingdom. Magic Kingdom can be covered in a day or two. Epcot is two days. Disney's Hollywood Studios and Disney's Animal Kingdom are a day each. You also have two water parks and Disney Springs. Touring the Walt Disney World Resorts involves more planning and there is a "pressure" to see everything so it can feel overwhelming.
If it was me traveling for the first time with a three year old, I would go to Disneyland in California. It is an easier trip to manage and you can add other Southern California experiences to the trip as well.
I would visit the Walt Disney World Resorts when your children are older.
Wishing you a magical day!
Beth