Hi Jennifer! Each agency/agent has their own submission guidelines. You can research each one separately. You can take a look at agents who rep authors with work similar to yours, perhaps. Some writers get a temporary subscription to Publishers Marketplace to see what's selling. Sometimes Google helps, too. And yes, it's safe to send your story to reputable agencies. Do not send if they are "closed" as that would be a waste of time.
Are you a scbwi member? They'll send you lots of useful info in this regard. You can also get a copy of children's writers and illustrators marketplace (cwim) at your library.
Definitely check out the link Vijaya posted!