Hoagie's Gifted Education Page has a Kids and Teens page with a variety of links that are grouped by topic. Here parents and children can find links to games, reading lists, contests, and many other resources that will provide enrichment for children.
Exquisite Minds has a great article on teaching critical thinking skills. There are suggestions for parents and links to resources for parents who want to learn more.
Byrdseed is also a great website for parents and teachers who are looking for ways to challenge students. While this site doesn't have a clearly defined portion for parents, there is a lot of information here that parents may find useful. You can also subscribe to a newsletter and to be sent links for weekly puzzles and language enrichments.