There are many useful links for math but here are some that your child might enjoy and can help them.
Cool Math
http://www.coolmath-games.com/
Adapted Mind
http://www.adaptedmind.com/
IXL
http://www.ixl.com/
Khan Academy
http://www.khanacademy.org
Prodigy Math Games
https://www.prodigygame.com/main-en/
Science
https://www.sciencenewsforstudents.org/