Point the little ones toward PBS Kids, the website of children’s programming through the Public Broadcasting Service. (My three-year-old has learned to navigate this user-friendly children's website by himself!) You can design an online treehouse or look for shapes in the clouds at It’s a Big Big World or learn about engineering with Curious George at Pogo-A-Gogo.
There’s a special page for older kids at PBS Kids Go!, and there’s a good selection of science and math activities here. You can design a WHOAHlercoaster (mine kept crashing!) or build a Geyser Surpriser at Fetch! or Crack Digit’s Code and play Pattern Quest (I love this game -- remember playing the game Mastermind?) at Cyberchase.