Ahoy, Priyanka!
Having a lanyard to hold your Key to the World card aboard the Disney Cruise Line ships is a fantastic idea! The KTTW card is the same size as a standard credit card. I have found that most typical ID holders will hold your card.
If your children are a little older, consider that they might also want to carry around their smart phones. My daughters now each have a small cross body purse that can hold their phone and their KTTW card. Since the Disney Cruise Line Navigator App can be loaded on all smart phones, they use the phone for everything, including a camera, while we cruise.
As for me, I tend to carry my smart phone, my KTTW card, a camera, and lip balm in a small purse while onboard. I also have a waterproof pouch for my phone, so I will put my Key to the World card inside it when we are swimming.
I hope you have an amazing cruise on the Disney Dream! We just sailed in July on the Dream, and it was wonderful.
Castaway Cay is truly an island paradise.
Happy Cruising, Tracy V.