We couldn't find a good solution to protecting our kids from the fireplace, so we made one. This is what I came up with. It is a very simple design of four pieces of wood screwed together. Cover that with foam padding or quilt batting to your liking. Then, if you can sew, make the cover and use a staple gun to upholster the bench. You could easily get away with just covering with fabric and stapling in place as well if you can't sew. The nicest part is it slides on and off fairly easily (not easy enough for the kids) when you want to use the fireplace and the bench makes for extra seating!
Total cost for wood, padding and fabric was around $50.