I believe Jill means Crown Center -- not Crown Plaza -- it's a common mistake, my hubby calls it Crown Plaza all the time and I've worked in that area for over 5 years now.
Crown Center also has a Clifford exhibit that runs through the end of the summer. Hallmark's corporate office is located at Crown Center. Kaleidoscope is within the Crown Center area.
There is a Hyatt that is within Walking distance of Crown Center -- there are skywalks that connect the Hyatt to the Crown Center shopping/dining area. A Walkway over to Union Station and Science City which is connected to the Westin Crown Center hotel.
Most of the shops are mid-to-high range stores, but are fun to browse.
I just heard that there is a trolley that will start running from Union Station to Independence, Missouri -- the home of former President Harry Truman. I don't know a lot about it, but do recall that it only costs $1, but I'm not sure if that's $1 each way or round trip -- either way -- it's still pretty cheap.
Kaleidoscope is free, but you should call or check their website to see if their sessions are open to the public -- kids do have to be of a certain age and they are usually looking for a parent or older sibling to act as a helper during the session.
Depending on which day of the week you'll be here, there are Free Friday night Flicks on the lawn at Crown Center -- you'll need to bring something to sit on though if you prefer not to sit on the ground.
Hope this helps!