I'm the parent of a 25 y.o. who has gotten in plenty of legal trouble over drinking and drug use. As much as it kills me to hear about the latest disappointment, I would be much more upset to find out through another source, e.g. mail from the court system, etc.
Please be upfront with them. They want the best for you and will support you no matter what, especially if this is the first time you've ever gotten in trouble with your use. Expect them to question you extensively and go through various stages of shock, denial, anger, etc. Be prepared to lay it all out there.
My best advice: make this a one-time occurrence. The more you repeat this cycle, the more bitter they will be and will eventually withhold support. You all deserve a happy, productive life.