http://www.factmonster.com/dictionary.html
  (took ages to open so I gave up)

www.onelook.com
(more for older students)

http://www.wordcentral.com/
  (the one I chose to use)

http://www.wordsmyth.net
 (my second choice)

http://merriam-webster.com/  
(dictionary & thesaurus - maybe a bit above Yr3)

http://www.macquariedictionary.com.au/
 (search facility gave no result for 'bombastic')

access to one of the "kiddie" writing software programs like Creative Writer the students may be able to use the thesaurus for your task.

You could try Word's thesaurus: a chance to introduce keyboard shortcuts like Shift+F7 or right click the word (it has to be in a sentence) and select synonym from the shortcut menu. (option if online not available)
 
http://www.rcls.org/ksearch.htm
(clean interface to search a number of sources)

Have a look at the range of dictionaries on page
http://library.trinity.wa.edu.au/subjects/english/dict.htm