Most of us first met the Mahabharata as a story too big to hold all at once. That's exactly right. It is too big to hold all at once. So the question for a parent of a 3-, 4-, or 5-year-old isn't really where to start. It's what to leave for later.
Start with character, not conflict
Begin with people, not battles. A toddler can fall in love with Krishna the butter-thief long before they meet Krishna the charioteer. Bhima eating fourteen ladoos. Yudhishthira being honest even when it hurts. Those are the doors in.
The war can wait. The dice game can wait. Karna's full story can wait. None of it is off-limits forever. It just isn't the first room in the house.
What to do when the questions come
Your child will ask. About who's the bad one. About whether Karna is good or bad. About why family fought family. These aren't interruptions of the Mahabharata. They are how the Mahabharata works on a child.
A good answer at age four is rarely a final answer. You can say: this is a story we'll keep coming back to. There are parts we'll understand when you're older. That's true of every great story, and saying it out loud, in those exact words, is one of the more useful things a parent can do.
The three-stage path most families settle into
Ages 3 to 5: Krishna's childhood, and the Pandavas as children. Friends, food, mischief, kindness. The Mahabharata as a family album.
Ages 5 to 8: Adventures and tests. Abhimanyu's courage. Eklavya's devotion. Bhima and the rakshasas. Stories where someone is asked to be braver or kinder than they thought they could be.
Ages 8 and up: The harder threads. The dice game. The exile. The questions that have no clean answer. By now your child knows the characters as old friends, and the difficult moments land in a child who has earned them.