Our preK-4 school subscribes to World Book online, PebbleGo and PebbleGo Next, and Gale in Context:  Elementary. Many teachers also use EPIC (online books).
In addition, I'd like to provide teachers with a list of websites for general searching, to discourage the "Just Google It" practice.  Which ones do you like?  I am happy to share compiled responses here.

Thank you
Elaine Shapiro
Broad Brook School