Why not? 

You are writing a personal, heartfelt letter, so feel free to use personal, heartfelt words. 

If you were writing a formal letter sticking to formalized greeting would be advisable (find posts on this subject [here][1] and [here][2] (thanks, @chirlu)), for personal use, these rules are rather loose – especially in special circumstances. Writing the infamous *mit freundlichen Grüßen* would – to my ears – sound odd.

As it stands, I see nothing wrong with your suggestion.  
Consider capitalizing *Euch*, though. It’s optional for *du/Du* and *euch/Euch* nowadays, but elder people will have learned that in letters capitalizing personal pronouns is mandatory.


  [1]: https://german.stackexchange.com/questions/5056/what-is-the-accurate-translation-of-best-regards-to-finish-a-letter
  [2]: https://german.stackexchange.com/questions/245/unterschied-zwischen-schriftlichen-abschiedsformeln