Is it a 2 stroke or 4 stroke? If a 2 stroke it should smoke, this shows that the engine is getting oil. If a 4 stroke, has previous owner filled the tank with 2 stroke mix, this has been known to happen. If it is a 4 stroke, and has neat fuel in the tank, and still smokes, it probably has a badly worn cylinder, and will need the head and cylinder removing to check for wear, but if you keep the sump oil topped up, the kids could use it "as is" for a while, to find out if they take to riding quad bikes.

it's a 4 cycle motor. I drained the oil and the gas and put clean oil and gas in it... I also let it run for probably 20 minutes to try to let it burn whatever could be sitting in there out. Is there a place that I can put seafoam in it to clean some of the much out of there and possible let the valves seal better?