For Mac OS users, there is a system setting that may not allow you to tab onto several types of elements in a web page. To change this setting:
www.sciencekids.co.nz is a great site full of interesting and fun science learning tools for kids to use.
www.brainpopjr.com is a fun website with several free options to use and you can also join it for a fee.
www.sciencemonster.com is a fun and learning site full of interesting science information.