You're probably pushing it. Allow an hour to get to WDW (I know, it's a 25 minute drive, but you need to get to ME or car rental)....and it will be rush hour. You need to check in at your resort, then get to Chef Mickey's. 10 minutes for check in, 30+ to the Contemporary.
Just a thought though...I normally suggest that people cancel ADRs if they think they can't use them, but.....this is probably near to the last ressie they have. Why not keep the ADR. If you get to your resort and are ready to leave for dinner and it is not past 8 pm, get over there quickly. If you are after 8 pm at your resort, just do a CS somewhere. However, call (or have the concierge desk) call them for you. We got "crunched" on an arrival time and weren't going to make our ADR. They said to come over as soon as we could and they'd hold it for us.
If you don't show up, they will give your table to someone else. However, if you call from your resort, the restaurant can tell you whether they can allow you to arrive late or not--so if they can't take you, you get CS. Just don't pump up the kids about dinner with Mickey on the first night.