From what you've said, the childcare payment arrangement was one between the two of you, and you have paid that off now, I am sorry to say but I think that you would have very slim to no chance of getting any money out of him for that. If you have no defined court orders, and if you are entitled to the full amount of CCB & CCR, then you should receive those government payments, leaving your out of pockets for the balance only. THAT is what he would pay you half of (if he was going to, which clearly he does not intend to).
Now the issue with the car seats - you could make a point and tell him that you are not longer willing to supply him with yours, and he needs to supply car seats for his own car for his daughters, and if he fails to do so, they will not be travelling with him. It is, of course something he ought to have done straight away, but you've managed for this long, and it may 'seem' unreasonable for you to now change your behaviour.
The Child Support debt IS something that you should be able to chase up, as it goes through the external agency - it sits now at 2 months, likely to become 3, but I can tell you that typically, the CSA does not chase debt until it gets to be well over $10,000 - however, if there is a CS debt, the monies are taken from a persons tax return (IF they do their tax returns!), so IF he does not pay up in time, you may receive the money at the end of the financial year once his taxes are submitted. ANY CS money owed is taken first, before he will receive his tax return payment.
I am sorry, I know this is not what you want to hear, but so many of us live in hope that the ex partner will 'come good' but many of them do not. Some men think that Child Support payments cover everything and no other money should ever be paid out for their own children, some men are better than that and recognise that kids have needs regardless of where they spend most of their time.
Start to budget as if you will never see any money from him, and know that if you do, it'll feel like a bonus, but you will incur the 50% MIT clawback from your FTB payments.
You are working, which is great for your family situation, it is tough, but it can be done. Each time you purchase things for your girls that their dad should be paying for, know that you are a great mum and a great provider for them, and you are capable of giving them what they need.