The short answer is NO. No, they are not too young.
I don't even have to know who you are or how old your kids are to tell you definitely that your children are not too young for Walt Disney World.
I mean, come on, it was made for them.
My Disney History
Earth. My daughter was about 6 weeks old when she went for the first time. And my son was one month. Obviously a newborn who can barely see past his own nose isn't being visually stimulated. But still, I think there are excellent reasons to take your munchkins early.
MommaStu's Top 5 Reasons to Take Them Young
1. Get the kiddos used to traveling. I had my daughter on the move pretty much immediately. She learned to nap when she needed to, and it did not need to be quiet for her to sleep. I never understood how Mommies got anything done when they had to be home from this time to that, or if the house needed to be museum silent for Baby to nap. That would have driven me bonkers. This also came in handy whenever we were at a Theme Park, and she needed to rest. If she needed a nap, she would. There was no rushing back to the hotel room, or worse, struggling with an exhausted, screaming child in the middle of the Magic Kingdom.
2. Raise genius babies. You know from my last point that I couldn't stand to be in the "baby bubble" for long. There is plenty of research out there that says exposing your tiny one to many sights and sounds helps aid brain development. Your baby's brain makes sound connections first, and there is music playing all over the Walt Disney World Resort in Orlando.
3. Travel babies are better suited to stay the night away from home. It's not really rocket science that if you never sleep anywhere other than your own bed for the first 10 years, it's going to be a big deal to stay in a hotel - I see one of two things happening:  your child is bouncing off the walls with excitement or  your child is terrified of the new space, sounds and smells -- either one would be a nightmare to deal with after spending a day in the hot Florida sun at a busy Theme Park.
4. Kids under 3 are FREE! Disney is expensive. There are ways to budget yourself and make your trip "more cost effective." This is one of those ways. So why not take advantage of the opportunity to get at least one member of your party into the Park for free?
5. Disney is a magical place, but when you are tiny, it is AMAZING! Everything is exciting... just being on the Property is thrilling to a young child. And there is no corner of the Walt Disney World in Orlando that has not been touched by Mickey Magic! My husband always tolerated going before we had kids. But seeing his kids' faces light up at every turn (yes, even after almost 50 trips), his heart smiles. And he almost becomes a giddy child himself.
Our Disney Sides
We are fortunate enough to live a short drive away from the Happiest Place on Earth! And we take full advantage of it!