Welcome. It can be quite overwhelming planning your first WDW trip, but luckily you've got plenty of time to research it. I recommend getting the Unofficial Guide to WDW for tons of info that will help. Our first trip was when the kids were 5 and 7. We arrived mid-afternoon, and after checking in, we just went to wander around Downtown Disney a bit and have a nice dinner there before heading in early. Everyone was tired from travelling, and we planned to hit the Parks before rope drop every day we were there, so sleep was important.
Yes, you will need an ADR for a place like Chef Mickey, and all other popular locations, especially at prime dinner time. FPs are for rides not meals (don't be confused by the temporary FP system they had been using at Be Our Guest for quick-service lunch in the MK - that's gone now). While some people like to check out the MK upon arriving even if it is brief, I might suggest that it might be more fun to see it first thing in the morning at rope drop when you can spend the whole day there and not have to arrive and leave with the throngs of people watching fireworks; while the fireworks are lovely, that might not be a magical start to your trip. Plus you'd be using an entire's day ticket just to see fireworks which is not terribly cost effective. You might want to consider booking dinner at Whispering Canyon at the Wilderness Lodge which is a lot of fun for kids, and then watch the Electrical Water Pageant from the beach there before heading in for the night. (Oh, and I strongly suggest you not book more than one sit-down/character meal per day for your first trip.)