Is there a company that organizes day excursions from Delft to Keukenhof?
I wouldn't know one, but it is extremely easy to do it on your own:
Buy the EUR 27,50 "combi-ticket" at the Keukenhof website for the Keukenhof Express bus from Leiden to the Keukenhof. The bus leaves from a clearly signposted stand in front of Leiden station, and runs every 10 minutes or so.
Take the direct Intercity train from Delft to Leiden, running every 15 mins (check at the website of the NS, the Dutch national railways). You can buy the train ticket from the machine at Delft station or on the NS website.
Thanks for the information!