Online dictionaries:
http://www.learnersdictionary.com/ An online English/English dictionary created specifically for English learners.
http://www.visualdictionaryonline.com/index.php A visual dictionary organized by themes
http://www.wordreference.com/ Amazingly thorough bilingual dictionaries for learners of over a dozen languages (There's also a great smartphone app.)
Websites for young children:
http://pbskids.org/ Quality early literacy games for young learners based on popular children's programs on public television. You can ask your child's teacher which skills they're working on and find games that are just right.
English idioms and expressions:
Educational resources for families:
http://www.colorincolorado.org/ A bilingual (English/Spanish) website for families and teachers of English learners from the Public Broadcasting Company
http://www.readingrockets.org/ Literacy resources for parents and teachers from the Public Broadcasting Company