I wish we had a LARGE supply closet. Some place to store large rolls of paper (on a cart), an overhead protector, basic craft supplies (as to not interrupt one class for last minute supplies). And large enough for a small table/work surface for a cutting system. Basically a very large closet/small room that can be locked.
I would include a large gathering area. But it also need away to be secure. Wouldn't it be nice to have a place to do small performances?
One church I visited had an enclosed counter area w/ a small swinging door, in front of the nursery. I think it was a kind of check in station.
Also make sure the design includes a way to add on (if the land is available on any side). Our church has a lot of odd connections and odd doors that don't line up w/ hallways, in each new addition.
Go visit churches in your area. I have found most CE people are willing to 'show off' their area. If you look at a church as large or larger you might get idea to grow into. If this addition helps to grow the church it will also grow in the amount of kids that come.
Do you need an office or workspace? Where do you store curriculum and children's files? Is there room for a computer and printer.
Also, what do you like and dislike about you current area? What do you do that stretches you currently?