I used to lead groups of kids for a living - at outdoor schools and day camps. Now I also live in Belgium (coincidentally) and am a long-distance hiker, so I hope my ideas will help.
The walking speed will vary based on a lot of factors, the biggest factor being to total amount of time walking you expect due to fatigue and boredom. Another factor to consider is trail quality and weather. I'm guessing from your username that you'll be walking in Flanders rather than the Ardennes, so that will make the walking times easier to calculate since the trails are flatter and more gentle.
If you keep each walk short (as you say, 30 minutes) then the group can walk at about 3.5 to 4 km / hour on flat ground; 3 km/hr if there are hills. So the distance for each 30 min walk could be 1.5 - 2 km or a little less. Maybe make the first ones closer to 2 km, and the later ones (when the kids are more tired) to 1 km each. I recommend singing songs or playing trail games to distract the kids who get bored easily, and trail snacks are a good idea. Set the expectations up front so that kids know what to expect.
I led a group of 6-year-olds on this walk on a weekly basis for a summer, and we called it the "death march" because there were hills and some kids in the back would be very unhappy about it. You can see it's 2.25 km long, so at 4 km/ hour it took us about 30 min - 45 min to walk the whole thing. The hills on this path were higher than those in Flanders, lower than the Ardennes.