Sorned car on jointly owned private land

Herzlos

If he has other land he doesn't want to park them on then you'll get nowhere with good will. As he owns the land you'll get nowhere legally.
Have you asked him why they are there? Is it just to store the material? Is it for a particular job?

Unless they are physically blocking You then there isn't much you can do except not let the kids near them.

Will he let you cone/tape them off? Kids should be capable of playing in a space without hitting a parked van.