My co-parent threatening to stop me seeing my 4 year old daughter after I said I was looking to change rear-facing car seat in my car to front facing.

You aren't thinking things through here. You go on about your perfect driving record as justification for 'turning' her, and then mention how your ex has crashed several times.

Once you FF your child in your car, it'll be harder for the child to accept still being RF in mums car. And yes, that's entirely your prerogative, but in reality it means your daughter will end up FF sooner in her mum's car, where according to you, she's less safe anyway.

You seem so concerned about tit for tat that you have stopped thinking about the best interests and safety of your child. You are also using issues of concern over safety and hygienist in mums home more as ammunition rather than trying to resolve them.

If I thought my child was living in an unsanitary home and her health was being compromised, I’d be straight onto social services. Why haven’t you done this OP? The car seat is the least of the issues here.

Your poor poor daughter and friend

thid needs a court ordered agreement - get legal advice and get it started clearly setting out expectations

and look at whether your daughter is safe

You don't need to announce you're changing a car seat as long as the car seat you switch to is the right one for the age/height of your child. Your first mistake was mentioning it.
You only have contact via a group chat and a friend indicates that the relationship you have is poor. In which case you only need to be speaking about essential matters. Don't offer up anything else in future.
For the sake of keeping the peace of stick with the RF car seat for now, as it is still safe to use. Other than preference there is no need to change and cause an issue now.

Going forward if you have safety/neglect concerns regarding your child health you need to be reporting those.

Otherwise only speak on a need to know basis from now on.

So you were in your existence house overnight a few weeks ago to hear the 'cat induced' coughing fit, but now can only communicate via a third party?

There seems to be a lot more going on here then car seats and a kid going in the garden barefoot.

The house situation sounds potentially dangerous.

I expect you are scared to report it but I think you should. Poor kid.

I don't think SS would do much with the living conditions, they allow many kids to stay in a much worse state.

OP, I imagine your child's mother has been got at by one of the crazy car-seat groups. Regardless, she can't restrict your access, but if it's not formalized, I'd get right on that.

How do you know what her house is like if you only communicate through a friend?

I didn't realise a 4 year old would fit into a rear facing car seat - both my grandchildren have front facing seats, 2 year old has one with 5 point harness, 4 year old has a booster seat with back support using the normal belt.

The state of the house I would reporting to SS

Surely it is his business if he thinks his child is at risk.
OP if you haven't got a child contact order get one so that contact arrangements are set down legally.
If you think your child is at risk with her mother apply for full contact.

@Chuchoter is the OP also 'awful' by the same vein, since he hasn't got court ordered access sorted and is making his child live with fleas? If the mother is so bad, the OP would be the resident parent.

She sounds awful and I doubt it's about the seat at all but more about trying to control you or find an excuse to stop you from seeing your daughter.

One of the reasons she doesn't want you to see your daughter is because your daughter is getting small injuries and allergies from her messy home and the child is getting older and more able to verbally communicate to you about the poor living conditions she has to endure.

Why haven't you reported to SS with your concerns?

Its not her friends job to be a contact between you

Grow up and talk to your ex

You're using unsafe conditions in your child's primary residence for point scoring against your ex, rather than out of genuine concern for your child.
For that reason you sound like a massive prick.

Is this your first visit to 'this wise online forum' ?

You need to get your access to your DD sorted properly.

You need to sort out some kind of communication about relevant things only with your ex-partner(?).

If its not safe for your DD tp live there, broken glass, fleas etc then why have you left her there and not fought to have her live with you?

Presumably she is the mother of your child? I’m not sure why to insist on referring to her as ‘your’ ‘co-parent’ to try and diminish her role.
Most of your post is tit for tat, what-aboutery. Presumably you’re grossly over exaggerating or you wouldn’t be allowing your child to live in a shithole?
The things you’re saying about rear facing car seats are proof that you aren’t the parent that has taken responsibility for researching the safest way of transporting your child. The mother clearly has. ‘A bit safer’- it’s actually 5 times safer. You acknowledge that they’re safer- why isn’t that enough for you? You say they cause sickness and vomiting, but don’t say that your daughter is suffering from either? And you say they’re uncomfortable as the child gets older- based on what? Many children comfortably travel rear facing well beyond 4. I suggest you do some actual research, watch some adac videos, look at the statistics.
Let me make a wild guess: you want to buy a car that makes a rear facing seat difficult?
You list a load of stuff to apparently try and prove that she isn’t safety conscious at home. For somebody with such strong feelings about all that, I’m confused as to why you’re trying to introduce something that is a reduction in safety? Are you parent of the year or aren’t you? Make your mind up.
And no, I’m not ‘your co-parent’/ mother of your child before you or anyone else suggests it.

To be honest you need to be putting your child at the centre of your decisions. Rear facing is safer statistically, if she is rear facing with one parent and not the other is that going to cause an issue for your child getting into one of your cars (some 4 year olds can be a handful). I’d simply be doing the best for my child in the circumstances, if you have a seat that allows rear facing at her height and weight I’d carry on with that until it’s outgrown, then make a decision based on your child’s height, weight and the latest safety advice when you need to buy a new one. All the rest of the stuff is not related to the car seat and should be dealt with separately.

If you always have your best child’s interest at heart make the decision based on how she is going to feel/be in any scenario -and try to take how you feel out of it as it’s not really supposed to be about you and any mediation or court style action will reinforce it’s what is in the best interest for the child

That's why it is important to have genuine context, which the OP will likely refuse to give. If this is a man that lives far away and sees the DC a few times a year, what he has mentioned here isn't going to drastically change the DCs living situation.

It may help to remove the ‘context’ issues from the discussion as tbh they sound like ‘yeah, but’.
Have you tried being curious, letting the co-parent know you’ve heard their concerns (emotions) and understand how a transition can be difficult . Maybe rather than trying to challenge with facts suggest a trial period to explore outcome.
carrots are usually more effective than sticks

God I just feel sorry for this child. The pair of you sound intolerable tbh. Poor poor little girl.

My wise words are SS for neglect or you apply for full custody.

Why haven't you done either?

"Wise words"

I wouldn't tell her about things like changing car seats in future.

Formalise your child care situation before doing anything nuclear so that you have a safety net in terms of access.

Then report her to SS and discuss with your solicitor what would be required to obtain sole custody