Financially ,every reply above makes perfect sense .But ,I was a young fellow once ,who fortunately had a father who had some tools ,and he saw that I had the interest and some ability to take things apart ,and put them back together again .My family was fed for many years by my interest and ability to repair items just like your kids mini bike ,and I learned a great deal of that by finding a piece of “junk” just like your boys dragged home ,and figuring out how it worked ,what I needed ,and where to get the parts and how to pay for the parts .I was able to sell my shop at 35 ,retire ,and do as I please ,while my own kids were not even in high school .That was before the internet ,which has made a project like that much much easier .
Sometimes ,a project like that ,might be a priceless opportunity for a young fellow or two ,even if it doesn’t make financial sense on the surface .