how to convert solidworks file to obj ?

Yes but Keyshot is around $1000 - $1500 and is not something everyone has or can afford. Also from the experience I've had, the quality isn't as good as other methods. You can use ScanTo3D if you have Solidworks Premium, or there is a nice little free script that Neil Larsen has posted on the Solidworks Forums which is my personal preference.
https://forum.solidworks.com/message/285747

convert solidworks file to obj

Answered with a tutorial: https://grabcad.com/tutorials/how-to-convert-solidworks-file-to-obj

can also be used in a similar manner by Accutrans
http://www.micromouse.ca/

This works brilliantly for 'single' parts. It doesn't seem to work with assemblies. Is there a way to convert the assembly in Mesh Lab to a single part - or is this best done in SolidWorks before the import?

Can anyone upload a .zip file of an OBJ export of this File? Working on a cool project for kids. Thx.

opensource FreeCad...
https://www.freecadweb.org/

I use online converte http://www.greentoken.de/onlineconv/

Or you can also try https://www.convertcadfiles.com/ It has a free trial as well.