An open textbook is an openly-licensed textbook offered online by its author(s). The open license sets open textbooks apart from traditional textbooks by allowing users to read online, download, or print the book at no additional cost.
For a textbook to be considered open, it must be licensed in a way that grants a baseline set of rights to users that are less restrictive than its standard copyright. A license or list of permissions must be clearly stated by the author.
Open access (OA) refers to online research outputs that are free of all restrictions on access (e.g. access tolls) and free of many restrictions on use (e.g. certain copyright and license restrictions). Open access can be applied to all forms of published research output, including peer-reviewed and non peer-reviewed.
Source: "Open Access" Wikipedia article
NOBA -- Mostly psychology, but some seem to relate to child development
National Academies Press -- These are free to read online & to download as a pdf but a printed copy costs ~$40 – scroll down the page to see all of them.
CK-12 -- Textbook resources for elementary schools.
OpenLearn -- These are free online courses.
Several online modules on Childhood health and development
This is a free child development lesson for educational purposes from AAAS but not OER so no adaptations (use as-is).