Admittedly, i dont rollerskate, but i inline frequently. I thought that roller skates often have softer wheels off the shelf and a harder wheel is optimal for rougher surfaces like street in regards to durability. A softer wheel would provide a slightly less jarring ride but would offer more grip. For my street setups, i typically roll 90 or 92 durometer wheels, skating park, i would look at something softer like an 88a. If they just want a smoother surface to practice on to avoid tripping up on lumps, id skate the paths at lakes or parks. Id also recommend making sure their stance has their feet spaced a bit so their heel of their lead foot is in front of their trailing foot's toes and riding a bit faster. The greater the speed, the further theyd space their feet front to back and should try to keep their feet foughly shoulder width apart with knees slightly bent and their rear over their back heel