Depending upon the location of your room, it's in the neighborhood of 15-20 minutes at a moderate walking pace. If you're headed to the Boardwalk itself, plan on 20 min.
The path is right alongside the water, and you will probably pass a boat or two along the way. It's not much different than walking through a theme park, but certainly not as stimulating for the kids. I ended up carrying my 3yo most of the way last time we did it, simply because it was easier than trying to prod him along.
There is also boat service that runs between MGM and BW. The boat could take more or less time depending upon how long you need to wait for the boat to arrive, but at least you'll be sitting once you get onboard.