If it's your car then you have the right to dictate how you handle car safety as long as it follows state laws. I would say in my vehicle I feel that your daughter would be safest in a booster seat. Because of the liability I face in an accident, I would like to purchase one for her, Here are a list of seats that work well.

If you are in DC or MD its the law until they are 8 anyway. Use that to your advantage.

I would just ask them to buy a cheap backless booster. Tell them that the seatbelt fit is not correct in your car. Or, see if they will reimburse you for one.

This one, incognito is nice as it doesn't look like a car seat.

http://www.amazon.com/Safety-1st-Incognito-Positioning-Black/dp/B00FLXBHIU

Does she fit fine without it in her parents vehicles? Some cars have belts you can adjust or are just placed differently. I would just tell them you'd like to use the booster in your car because the belt is rubbing on her.

Find a well-researched and concise article online, forward it to the parents with 'hey x, I just saw this. Should we keep Gigi in a booster seat still?'

I think the child I drive should be in a booster seat or something like it. She is 7 years old and used to sit in a booster seat but does not anymore. I was checking that she was bucked the other day and noticed that the seat belt is pretty much on her face! I want to get her back in a booster seat or something like it but don't know how to approach this with the parents. Any ideas? Thanks guys!