Unfortunately, the best and easiest are not always compatible. :-)
For it to last you need to have some type of border, a weed barrier and then the topping. Depending on where you live you might get crushed granite for the topping. It looks nice, it's easy and maintenance free and it has a nice sound as you sashay to the garden. I'm not sure if it would be nice for the scooter. It will depend on the size of the wheels. I would dig out the lawn about 4" deep at the width desired. Lay down a good weed barrier, don't get cheap here, it will save you a lot of labor in the future. Then line each side with brick running vertically. Then fill with crushed granite or other simple topping. It may need some tamping to get a good smooth surface and compact the material for longer lasting service. Now, get a glass of iced tea and sit in the swing and watch the little tyke enjoy the newfound route.