LITERARY ARCHAEOLOGY

Something caused me to think about this book I read way, way, way back in the day whose title and author are both lost to me. I don't know what to call the intended age range. If you were reading Encyclopedia Brown or The Boxcar Children, it was probably right in there with those. Here's what I remember of it:

- A kid falls through some sort of dimensional gate behind a junky toy store(?)

- Ends up inside a parallel universe where there are a bunch of alive toys

- The toys are engaged in a war with -- I swear to god it was giant seagulls
-- There's a scene that involves a hot air balloon and an attack by the giant seagulls, and I think the kid gets captured at one point

- Some sort of malevolent force in this universe is causing all the toys (and maybe the seagulls) to fall apart and die
-- I think it manifests itself as a gigantic hole in the ground or something

- At the end the kid makes it back to our universe

What is this book called and who wrote it, because I have to know. Does anyone have any idea what I'm talking about.