I would just tell these children, in absolutely crystal clear terms, what you want/need them to do.
"Get off the car. Now. It is time for you to go home. Now." This can be said sternly, but politely. The good news is that they haven't been on your car since, so maybe the parents *are* saying something to the kids in private.
When the little girl is calling to you, respond once, politely, but make it clear you aren't out in the yard for a conversation. "Hi, Little Girl. I'm busy right now, so I can't talk." Then ignore her.
Just let your dog out as usual. If Little Girl complains again, tell her, "You have your yard to play in. My dog has his yard to play in. There's a fence so that you and the dog can play by yourselves."
You might also start telling them they aren't invited on your property. "It is time to go home now. Please don't come over again unless we invite you."
If your SS is playing with the other neighbor child, "SS and Susie are playing now. It is time for you to go home. Please do not come back unless we invite you over."
No explanations, no excuses. Just the facts.
And if they don't go home, go over to their house, get an adult and tell the adult the children must be removed from your property now. "Hi. Your kids are in our yard again. Please get them and take them home."