From my observations in homes and classrooms, one of the more common strategies that children use when they encounter an unfamiliar word is to Google "[word] definition." For some time this strategy has led to Dictionary.com, as that is the website that is best optimized for search results. It is not, however, the website that is best optimized for giving child-friendly definitions.
Do you remember the experience of being a child and looking up a word in the dictionary only to end up more confused? For example, reading that discomfiture means "the state of being disconcerted," is of little help if you aren't familiar with the word disconcerted. Still, understanding word meanings is important to reading comprehension and research skills are important part of academic independence.
It is a good idea, then, to point your child to a reliable online dictionary for students. Below are a few I like. If your child uses an internet-enabled device for homework, make sure to bookmark at least two good dictionaries on that device (in the event that the definition in the first dictionary is confusing).
This is a resource provided by Oxford English Dictionaries. It skews primarily toward British English, rather than American English, so children may have trouble finding certain American spellings. It is a good back-up website to use if a student is having trouble understanding the first definition he looks up.