I have just been climbing Mount Nanos in Slovenia. It wasn't a very long trip, but we parked by the highway and walked to the top on the steep way (approximately 2,5km) before we walked along the top and then walked the softer and nicer way down. The entire trip was a bit less than 7km, but it was quite tough, and considering that I read several places that it was a family-friendly trip, I was kind of surprised by the challenges along the way.
Of course, we walked it in more than 30 Celsius, meaning that the sun was frying our heads and bodies along the way, meaning that we were sweating a lot and used quite a lot of sun tan along the way. All in all, we survived the trip and I can warmly recommend it to anyone who loves a nice trip in the mountains. It is kind of demanding, but kids above 6-7 years with some practice should be able to survive the trip (at least my 6-year-old girl did). But, be aware parents as well, it can be quite demanding, so you should be in quite a good shape yourself before you start the trip!
Have a wonderful summerday everyone!