I am looking for recommendations for workbooks or websites that help improve vocabulary. Primarily for my 4th grader, it would be good if it included material for my 8th grade student also.
I don't have a specific website, but the more they read, the better their vocabulary will be. Also, the more varied types of materials they read the better their vocabulary. Do they like to read? Could you help guide them to read some fiction, some poetry, some non-fiction, etc? Also, get them a good student dictionary so they can look up unfamiliar words as they read.
I agree with Momma Bird. The more they read, the better their vocabulary. Luckily my kids LOVE to read. Does your local library offer a summer reading program?
How in depth is the reading your kids are doing in school? There's a test my kids take regularly at school (they have an excellent reading program, each child in the primary grades goes to reading class for 45 minutes every day). The test is called Scholastic Reading Inventory. It graded the child according to the grade level the child is on. For example, a 4th grader reading on grade level would have a grade equivalency of 4.2 (4th grade, second semester). Their reading classes are with other children (aged the same) who are reading at the same level.
I'd also check with my teachers (school's not out here yet). My 4th grader took part in the UIL Spelling contest & she was given a list of words that had previously been on the test to study. Maybe a list from a spelling bee if something like UIL is not available.
Make up some fun games related to words. When we go on long car trips and see a billboard or other signs, we use the words there to see how many words we can use from them. Like Anagrams.
I just did a quick google search & came up with bunches of sites - I searched for vocabulary help.
Here is a link that might be useful: Vocabulary.com