Short story collections are a hard sell. Anthologies are easier to sell because usually there are one or two well known authors in them. There are many magazines that take short fiction for MG and YA. Even adult mags in the sci fi and fantasy genres will take short stories for YA readers.
Why not Google publishers who accept short story collection and see what comes up? Look also at who the authors are and how well known they are. I Googled MG short story collections and came up with a couple but whether they'd take your work depends on theme.