Cyclones30,
As the others have indicated, you have lots of great choices. IF your Mom goes along, I'd be inclined to go with the recommendations re: options on the northern tier of lakes in Quetico as they typically involve less work to access them.
I, myself, have a strong bias for that NW corner that BC & ST have already commented on. However, the NE side should not be overlooked. You can paddle & explore Pickerel Lake for days and still not see it all. While there may not be as many pictographs or high cliffs, there are probably more sandy beaches and family-friendly campsites. Also, there is hardly anywhere you can go within the entire north side where all four species of fish are not present. Re: Pickerel Lake, you can enter via one portage from a Nym Lake permit. It's even easier access from French Lake (no portage) or Stanton Bay (just the haul from the parking lot down to the bay) on a Pickerel Lake permit. If you use Stanton, however, you would need a local outfitter to shuttle you in. Non-resident parking is NOT permitted.
One heads-up... Pickerel Lake can become mighty surly in a strong west wind. Whenever I enter from the far east end, I do it EARLY in the morning before afternoon breezes kick in. While this should be somewhat of a consideration everywhere, it can be particularly important where those long, broad west-to-east reaches of big Pickerel Lake come into play. Many folks prefer to access the lake from Stanton Bay for that very reason.
There are lots of great campsites and beaches all around Pickerel. In late August and especially into September the people are mainly gone and the water is still quite warm for enjoyable swimming.
When you get down to planning for whichever area of Quetico you decide to venture into, feel free to solicit members on this board for campsite and fishing recommendations. The fish can become a bit more "scattered" later in the season but some of the veterans on this website can certainly help improve your odds of locating and landing them. Likewise, should you require outfitting assistance in the form of shuttles or equipment, we can likely come up with good suggestions, based upon where you choose to enter the park.
Good luck!
Jimbo