How do I tell my kids I'm going away for drug treatment?

I'm a stay at home mom of a girl 9 and a boy 5. I will be gone for 9-12 days for treatment. Don't think they will understand 'drug treatment'...what should I tell them so they won't worry?
Thanks Lynze83! You are right on with NOT using 'sick'...and the idea of write'n to them...THANKS So MUCH!!!

: http://alcoholdrugstreatment.info/how-do-i-tell-my-kids-im-going-away-for-drug-treatment