It's not a well-known word. I don't know it and I have a pretty good English vocabulary. I am going to have to check a dictionary to see if it's a real world or a made-up word. I suspect it is made up. It's not in ahdictionary.com. And it is not in my Kindle app that is supposed to be Webster's Third International, Unabridged. The only dictionary that is more complete would be the (full) Oxford English Dictionary, which I don't have. However, Wikipedia has an article, https://en.wikipedia.org/wiki/Imaginarium which says it is a name for "a place devoted to the imagination" and lists a number of children's museums with that name. So it's a new coinage and maybe it will catch on and get added to the dictionary someday.

"Imaginarium" is "a place devoted to the imagination." The word is like adding the "ium" in "aquarium" to denote a place, to the word "imagine".