"Make this" projects geared to different age groups. Also try the main site, Instructables, for DIY projects from knitting to gadget-making, crafting to engineering.
Classics for Kids --learn about composers, instruments, and musical terms; play rhythm and other games, and more.
Khan Academy: online course on more than you can imagine! Choose "Courses" to see a list of current offerings. Also try Khan Academy: Kids, designed for 2-7-yr-olds.
exploratorium: tools for teaching and learning--a wealth of resources on a variety of subjects, with a lean towards STEAM
CRASH COURSE KIDS : "Science with a Side Of Awesome" is this YouTube channel's motto. Many will be ready for the channel geared towards older kids (with far more content and on more subjects): CRASH COURSE.
Authors Everywhere! Children's authors and illustrators provide read-alouds, writing tips, art demos, and more to do at home. New videos added regularly.
Science Buddies --tons of shorter STEM activities as well as science projects in all branches of science, beginner to expert.
Science Friday: Educate --STEM experiments, projects, and lesson plans searchable by grade and duration
Jet Propulsion Lab --Launch rockets, build robots, explore your world and beyond! Activities, toolkits, and contests.