Preschool picture book series

I typed 2 paragraphs before I came back here to give you a QUICK answer: CERTAINLY let them know that you're working on a series. Don't forget that they make their money from YOUR royalties. If they know that there are other books in the pipeline, they'll be more eager to bite.

But don't flood them with material. One finished book + an outline of where you want the series to go. If you're writing a serial (book 1 should be read before book 2, etc.), you HAVE to send book 1, because if that one doesn't grab them (and your intended audience), they're not going to bother asking for (or buying) book 2. If you're writing a series (where the books can be read in any order), send your best one. DON'T send the finished one and write "I'm working on one which is going to be much better than this." Finish the better one first and send it.

Are you writing AND illustrating the series? Or have you already got an illustration collaborator? If the answer to either question is "yes", your best bet is to (be ready to) attach a .pdf file of the whole of one book, and write in the covering e-mail that a series is planned and - in fact - already in production.

Check the web-sites of the agents that you want to write to and follow their instructions. Some want nothing more than an enquiring letter to start with. They don't want the additional "weight" of attached files in their inbox. Your enquiry letter has got to "sell" the idea to them: make them ask to see a sample of your work. Others want to see a sample with your first e-mail. Sending a synopsis + 3 chapters of a 90,000 word novel is one thing, but you don't send a synopsis of a picture book for 3-5yo. A synopsis of a whole SERIES is a different matter. (Your Q makes clear that you know this: I'm adding it for other writers who might look in.)

I'm writing this with my publisher's hat on: that's what I'd like to receive from a new writer/illustrator.

Any chance of a link to your work?