If you decide to sue, you need evidence of the damage (e.g. pictures of the scratches) and also evidence that the children did the damage--if you don't have pictures of videos of their bikes hitting your car, then you can use testimony (e.g. your testimony that you saw them doing this), but in that case, if they and/or their parents deny matters, it will turn into "he said, she said," where who is more believable or credible will be critical to the outcome. Note that you need some direct evidence--either pictures, or the testimony of someone who actually saw the damage;  or perhaps paint flecks from your car on their bicycle handles or peddles. Without direct evidence, if all you have is supposition--i.e. the scratches look like the scratches left by bicycles, and you've only seen those children ride bicycles in the driveway--you will most likely lose, because supposition does not "prove" that they did the damage; it only makes it possible.