Ok, the book is set in a hotel or apartment building. The building has no 13th floor. But by drawing a hatch on the floor of the 14th floor, the kids can access the 13th, which leads to all sort of fascinating adventures, such as to the Milky Way. Pirates?
Memories are a little vague.
anyone?
Looks cool, has pirates, 13th floor, etc. But I read mine in the 1960’s, so this is not the one. I might buy and read anyway…
Ah, I was gonna ask about year read/published; that’s kind of critical information.
The whole “no 13th floor” thing isn’t an uncommon trope.
How about The 13th is Magic by Joan Howard?
I can’t find a good synopsis online
best I could do is the sequel
Hmm, that* could* be it. The fact that it is OOP does explain why I have never found it again.
Of course since it is OOP, I cant buy it and verify it. 
thanks.
I saw that book when I was poking around yesterday, but aside from the 13th floor thing, none of the details matched with what was in the OP (no pirates, no outer space) and the kids don’t draw a hatch on the floor AFACT; instead the cat leads them to the 13th floor which is accessible only on the 13th of the month.
A story called “The Thirteenth Floor” was printed in the January 1949 edition of Weird Tales, but I doubt it’s the work that the OP remembers. Still, I read it once and just spent about fifteen minutes Googling it, so I figured I’d share the fruits of my labor.