To ask MIL not to use their car seat?

Cot bumpers are unsafe yet available
Cigarettes are unsafe yet available
Alcohol is unsafe yet available

Forward facing car seats only have to he crash tested to something like 30 miles per hour. They are not the safest option for small children - rear facing as long as is possible, 3,4,5y plus absoloutley is multiple times safer than ff.

I'm following the law. If it was so dangerous to forward face then they wouldn't sell them for toddlers. I'm not rear facing her until she's 6 years old. I'm not arguing because I'm following the law and she has a safe car seat, thank you very much.

www.gov.uk/child-car-seats-the-rules

It is true though. If it were not, then forward facing car seats would not be available to buy.

To be brutally honest, you're risking your child’s life by taking them in a car full stop! It’s all about reducing risk. Factors such as reducing speed limits, manufacturing cars with greater safety in mind, higher penalties for anyone caught using a mobile phone in a car will all help to reduce the number of collisions in the first place.

How good is your MIL's driving? And how capable is she of clicking your DD into her seat properly? And they probably bought one that she can lift in and out of the car.

Can you talk to her about these aspects?

Has she yet had reason ro use their seat? Will she, any time soon? I'd just not bring it up at all, avoid any chat about them having dd alone etc.

If they end up having her in an emergency, just drop her off, insist on installing the rear facing seat in their car, and say "this is a non negotiable for me". They can judge you all they like, it's your dds safety you are prioritising

It's really not, please don't give information unless it's true. You can do however you wish, however it doesn't mean it's safe because it's 'a new seat'

For me, part of the equation would be how often she transports the child, how far and what type of roads. If they are the sort of (relatively) low risk infrequent journeys I am envisioning, I would probably accept that any increased risk is negligible. If she is likely to do longer journeys, often etc, then it is worth speaking up.

Dc rear faces until 4 and now FF which I don’t like but my husband wanted.

DC2 is 18 months and rear facing. It makes me feel happier. I watched all the YouTube videos about increased chance of breaking their back or neck

Rear facing is safer and my 3 year old still rear faces in my car. However, she has been forward facing in my parents' car since she grew out of the baby car seat. I know it just takes one acciennt, but they only use it occasionally (once a month maybe) and for local journeys so I don't have a problem with it. Rear facing/360 car seats are big, heavy and often more complicated to install, so not the best fit for grandparents to be lugging in and out their car. I think you are being unreasonable. However, if it's. Ah I'll you are willing to die on I'd think your best bet is just to make sure there is no need for your mil to have your child in her car.

Send her the crash test video of an 18 month old in a forward facing seat. That will help her understand

That's not true. It's not illegal to FF but it isn't as safe

It depends a bit when you say it. If it’s early enough that they can return for a full refund, then potentially.

If it’s already been accepted then it’s a bit of a dick move to shift the goal posts.

It’s also worth putting the car seat safety info in context. There’s some selective approaches to using statistics to promote extended rear facing. Not that they’re bad, but the difference isn’t as big as often appears. Things like no hands free phone use, tyre tread etc make more difference.

Research it. It it NOT safe for an 18 month old to forward face. My child may prefer it. But i prefer them alive. They will be rear facinf until at least 5 years if not 6.

You are risking your childs life by forward facing so young.

I was the same. Sure both my DM and MIL thought I was being precious but I didn't care. I bought them car seats to keep but they hadn't bought their own so it was easier.

You could fein that you've just found out about the the importance of rear facing, send some crash test videos and just send a message saying we know you care about keeping dd safe as we do so going forward you'll install your car seat in her car. You can always tell her she can use her seat when she's older but no doubt she's bought a crap team tex seat with white clips in which case bin it!

And you are right about RF. It's 5x safer but of course you'll have people saying you're overreacting and their kid was fine

Yes, speaking as a paediatric health professional, children should travel rear facing for as long as possible. Definitely until 3/4 years, longer if possible.
I understand it may hurt her feelings, but ultimately as her parents you have final say about her health and safety.