If the left side runs true (ish) and the right is still wobbly to a greater extent - then the rim cross section must be inconsistent. If this is the case then it will be very very hard to get both sides running within the same tolerance. Like xenologer says - make it work "well enough". The kid isn't looking for a TdF win on it - probably just wants it to roll down the driveway and do a passable skid at the bottom.