I am not sure that this outing qualifies as a hike, but it a beautiful relaxing walk through the Gorges du Fier.  It is accessible to all ages as the "trail" primarily follows a wood and metal walkway built along the edge of the gorge walls.
The only reason it is not handicap or stroller friendly is because it requires a few sets of steps to complete and the parking area is about 3 flights of steps above the start of the walk.  Also, toward the end of the walkway there are some areas where you walk on the natural terrain (rocks/dirt), however this terrain is very stable.
There are a number of informational boards along the way describing the history of the gorges, myths and legends behind the gorges and plaques showing what look like "faces" that have been naturally carved into the walls of the gorges.
The informational boards are in multiple languages, including English.
We completed this walk with two 3 year olds, a 5 year old and a 6 year old.  It took us just over an hour to walk the entire trail including taking short breaks on benches for snacks and to read the plaques.
Lucas, age 3. Eliana, age 6. Mommy, 6.5 months pregnant.
The children were very intrigued throughout the walk and didn't need any encouragement to keep moving.  The scenery and environment were interesting enough to keep them moving along and enjoying the outing as much as us.
They enjoyed looking at rocks, picking wildflowers, and admiring the scenery while trying to find forms that were naturally carved into the rocks over hundreds of years.
This walk was just under 3 kilometers (including walking from the parking lot).  At the start of the walk there is a small cafe with seating.  Before entering the trail you must pay for entrance.  Children under the age of 7 are free and adults are about 6 euros.  For official site information, including pricing and opening times, click here.
For full photo album of our trip to the Gorges du Fier, click here.