Selecting objects that are path constrained to a spline

I have several splines on a city landscape with VRayProxy cars attached to it. these cars have also headlight objects as children to these. Do any of you guys know how to back select the attached objects to the spline?

the (ideal) work procedure would be:
1. Pick Spline
2. Run Script
3. Get the selection of the objects that are constrained to the spline, + children

This would be pretty useful if you are path constraining several thousand cars into different splines in a city landscape. that would allow you to copy a street traffic by selecting the spline + the cars constrained, cloning them and then uninstance (making unique) only the cloned spline.

Of course this would be able to be done using wiser object naming, but having already a pre-named file makes it kind of complicated.

I hope you can help out,