I can give this a go if you'd like to send me the original image at as high a res as possible. I can comp out the cars and if it looks like there might be any issue with the replacement areas I'd look at bluring the further away section of the backgorund, effectively adjusting the images depth of field. This should still look natural and mean we can get away with more manipulation on those areas and will further emphisise the kids as the focal point of the image.
PM me and I'll send you my email.