How do I go about getting into the engineering field with only a design degree from almost a decade ago?
You have a Bachelor's already, which already makes you nominally qualified for graduate school! What comes next is figuring out how to make an entrance into the field when you don't have a background in it. The Bachelor's system currently in existence generally requires a basic education requirement, so I wouldn't suggest going for a B.S. in physics or engineering. I would instead suggest one of two things, combined with a healthy regime of self-teaching:


Simply take courses at a local institution
Get an associate's in physics or engineering


Would this make you a non-standard applicant, having your most recent degree be an associate's, or a bevy of post-bac coursework not for a degree? Well, yes, but you're still qualified. You're really quite young within the camp of non-traditional, older students. If you can back up a shorter (cheaper!), more focused set of courses with some experience doing applicable work (volunteer at a lab, try to land a position as a tech at a nearby school), that makes you just another applicant, albeit one with very different life experiences. 

There is a guy in my research group who has a wife and kids who transitioned into engineering when he was 30, and he swept right into an M.S. and then a Ph.D. program without getting a transitionary degree after eight years of no classes. It's definitely possible for top programs, as we're at a Top 15, but I would advise trying to get some experience and insight on the field at local institutions.