I think accepting it is the right choice. I don't think however I'd mention while accepting that you plan to keep looking for full time. I'd think that you want them to think that they're awesome and your one and only goal is a full time job with THEM. Now, if they happen to ask you if you'd keep looking for a full time job, that's a different story. I'm not encouraging you to lie to them. I just mean I don't see the need to bring it up if they don't. The last thing you want is to mention it to them without them asking and then having them think you're not really that interested in them. They're not idiots, they know you want a full time job, I just don't think telling them you plan to keep looking is necessary.