Pet Etiquette

Lily • 💕20 and pregnant with our first baby! I'm a bit of a princess at times💕
We have a little dog who is super sweet and friendly. She suffers from a disorder that effects her breed where their brains are too big for the skull and can result in phantom pains and other issues.
 
Some days are really bad, but she's usually fine.
 
The issue we run into are children that want to pet her without asking. Since she's usually fine, she is always excited to have some petting. On bad days, rubbing around her head can be agonising for her and make the day worse. She of course has medication, but it takes a little while to settle her.
 
Most kids ask before petting her, which we always commend the kids and parents on ("it's very polite of you to ask, thank you") but some run up and pet her violently. We have occasionally said something to the parents ("please can you make them stop, or ask first") and parents have been stubborn or rude in return.
What's your policy as a pet owner or a parent?
 
- Ask first, always?
- If it seems like a friendly dog, it's fine?
- Or not allowed whatsoever? 

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