Hi Dina,
Having stayed at both
Disney's Contemporary and
Disney's Beach Club Resort, Beach Club would win hands down for my family.
First, you can walk or take a short boat ride to Epcot and Disney's Hollywood Studios. This means no collapsing a stroller and easy trips back to your resort. While the monorail is fun, you are still waiting on it to get yourself back to the resort.
Second, the pool at the Beach Club is like a mini water park. If there is one requests for kids on vacation, it is to swim in the pool even when it is 30 degrees. The Contemporary has one of the smallest pool areas.
Finally, if you are looking for a relaxing way to spend an evening, the Boardwalk area and
Fantasia Garden and Fairway Miniature Golf are right out your back door.
However, if you plan on spending most of your time at Magic Kingdom, the Contemporary might be a better choice.
Wishing you a magical vacation!
Beth