I'm furious and not sure what rights if any I have to do anything. ExDP continually drives with DD on only a booster and occasionally (I believe) nothing. She's 4, she needs to be in a car seat. I feel like he's putting her at risk by doing this but feel I have no control. What can I do? Can I refuse to let him take her without one? But then when she's there i can't do anything. FWIW I'm very supportive of their relationship and have never refused contact, this is a genuine concern not a ploy to try to keep her from him although I suspect he'll play it that way. I've tried so hard to keep things amicable for DD.
If she is not sat on anything I think you have a right to do something about it (although not sure what, maybe report to police). But if she is the right height /weight for a booster cushion I don't think there is much you can do because at the moment they are still a legal way of children travelling in a car. Yes a high back booster seat would be more safe but then it is personal choice.