I feel very ignorant, because I'm not sure what the hearth is or if I'm thinking of the right thing but I can tell you what we did.
The ledge in front of our fireplace is about 12 to 18 inches off the ground and is nice and smooth and an easy place to sit down, especially if you are less than 3 years old. We have boys who love to climb, so this was definitely going to be an issue.
We measured it and bought a plank of wood that would fit on top of it. We also purchased a strip of carpet (from Menards, but I believe Lows and Home Depot would probably have this option as well) and wrapped it around the wood and stapled it to the underside. Then we simply set the piece on top of the ledge.
It has worked wonderfully for us. The wood and the carpet have really cushioned it, and our boys (now 3 and 5) have never gotten hurt on it - knock on wood, ha, ha, ha.
Anyway, it worked for us :-) We did the same thing for my parents, and it has been great there as well.