I went to Tamar in August with my family. We have older kids, but the path is suitable for kids from 4 years up. We started in Planica. This is the biggest Slovenian Nordic center with lots of new ski-jumps. Unfortunatelly this year there is still a hudge building place, but you can see 6 new ski jumps and the biggest one called Velikanka, where there is a world cup each year.
From Planica there are 2 roads. The left one is more like a road, suitable also for bikers and maybe more for winter. The right one is more like a walking path, suitable more for summer, since the most of the path is in the shades of trees. Both are suitable also for smaller kids. There is around 1 hour walk in each direction. We took the right one, since it was quite hot. In the beginning there are some stipper hills, but later on quite an easy walk all the way to the Tamar. On the way you have some look-out points, through were you can see the mountains around. Also you can see some boulder rocks with nice wooden bench aside. It is good to take enough water, since there isn't any all the way long.
Once you get to Tamar, you can see a beautiful grass livada, with a mountain hut at the end and beautiful surroundings. Tamar is also a starting point to a lot of mountains around (Jalovec, Vršič, Sleme, Mojstrovka, etc.). On the right side of the valley you can also go to Nadiža Spring, just climb for about 15 minutes.
The hut is opened all year around. They have also around 50 beds, but it is good to book in advance. For eating they have mainily local food, like soop, stew, very good apple of cottage cheese strudel, dumplings, mushes and curdled milk, etc. If you need something more strong to drink, try schnapss from pine springs.
After a good snack or lunch, there is again 1 hour back of easy walk. So all together you can have a nice half day trip.