We backpacked into Heart Lake last July when my daughter was 7. The trail is ~2 miles pretty much all uphill if I remember correctly. A lot of it is in the burn so there are many downed trees across the trail that you have to climb over. There's also a stream crossing near the trailhead, which my daughter had no problem with, and one eroded section of the trail that she called "scary".
The next morning we hiked up to the Six Mile lakes as well, which is all off-trail. That was about her limit at the time. It was a great trip. We had the entire place to ourselves.
As for the fishing, Heart definitely holds some (probably brookies), but we only had one bite that we couldn't land. We fished the upper lakes also but they all appeared barren.
Lots of photos here, if interested...
https://picasaweb.google.com/114149465768664324112/2011_ID_HeartLake