20 kilometers = 12.5 miles: I am seeing allot of advice given here to solve a potential problem. The Schwalbe Marathon Greenguard tires may be adequate for this situation. I would let the child ride with those tires to start with. The phone and a backup plan so somebody can help the child, are a must. As mentioned, teaching the child how to repair a flat with having the tools and at least a spare tube are essential. Tougher tires, tire strips and tire liners are options that will add differing degrees of extra effort and handling differences. I mentioned that I had used slime and finding out it was a problem went to Tuffy strips. I am gearing up for a trip and have decided I don't care about weight as much, so my order from Bike24 that took a whopping 28 days courtesy of customs arrived yesterday. In addition to the Tubus Grand Expedition Rack: the Schwalbe Pickup 26"X2.125 tires for the trip arrived. Alee from Cyclingabout is using these tires currently.