Hi M.,
The Kids Table is what you're thinking of probably. It's in the Wicker Park area. They have classes for kids as young as 2 (not sure about what to do with the 1yr old and 18 month old, but the older kids would probably LOVE this).
http://www.kids-table.com/classesforkids.php
I know there is another place in Lincoln Square called the Chopping Block. I've been there as a shopper, but not to attend any of their classes.
There is also WishCraft - a sewing place that has kids classes - it's in Roscoe Village... i've not been there, but I have a friend who has taken her 8 year old there.
http://web.me.com/wishcraft/Site/workshops.html
Also - The Joanne's craft store (on Elston across from Target) will have a schedule of classes. My daughter has taken a bunch of these and has really had a good time. They are frustrating to book (very disorganized, don't seem like they know what they're doing) but the actual class is REALLY well put together by the staff and very organized.
LillStreet Art Center has family workshops that are AMAZING. My daughter attended a b'day party there and has done a couple "out of school" day workshops. I'm not sure what they will have in April, but you can check them out!
http://www.lillstreet.com/
Hope that helps!