You are very vulnerable in my opinion, it's NOT just if you split it's also if he becomes incapacitated or dies - happens more to younger men than people realise
Taking certain points individually
In terms of earnings, he is main wage earner and self employed. The business is doing very well.
HIS business is doing very well unless you're a shareholder? If you were to split you'd gain nothing from the business if not a shareholder and if he were to become incapacitated and unable to work the business I'm assuming would fold?
I went part time 3 years ago when we started a family so my income is now less than half of his.
This is one of the main reasons why women are better marrying BEFORE children. Having children through sheer biology impacts women's earnings negatively in most cases. Add in childcare costs, childcare responsibilities (it tends to be mums who take time off when the kids are sick etc), loss of promotion prospects or even major career process basically reversing and you're incredibly exposed!
We have a joint account and have done since day 1
I had a joint account with my ex, days after I kicked him out for cheating he emptied the account including child benefit and child tax credits leaving me broke! Within weeks run up an overdraft that I was jointly liable for - all resolved during the divorce eventually to some degree (still messed up my credit rating for years) BUT if we'd not been married I'd have had no legal recourse.
"He'd never leave the kids short" - famous last words that I thought myself at one point to my detriment! Don't be me!
I pay for the childcare from my part time wage (so, most of my wage gone) and pay for the kids clothes/birthdays/Xmas etc. He pays for everything else for the house.
Wow! You earn less than 50% what he does yet I think you're possibly paying more than 50% of total family costs?! Why?
We have life insurance policies placed in trust naming each other.
Which he can change without telling you at any time and you could not find out until he dies. Ditto wills - while unmarried you can change it at any time without informing anyone.
Pension wise - I have a local government pension which he & the kids are named on. He has nothing being self employed being self employed is no excuse for no pension! That's really irresponsible of him! Re naming beneficiaries - again can change it any time he likes
Cautionary tale - relative was in a cohabiting relationship and had dc and was a Sahm. Partner died very unexpectedly. House, car every asset in his name, no will, not named on dcs birth certificates etc relatives "in laws" she'd previously got along well with took the lot! She had to go back to work full time within weeks of the bereavement. Awful situation.
As an ex nurse I've also witnessed horrific disputes regarding medical care and life or death decisions.
While there's no legal definition of next of kin (I think there should be) generally speaking when it comes to the crunch medics will tend to cover their arses and go with the person most closely legally linked to the patient if there's conflict.
You can get close to replicating some of the legalities without marrying but it's never exactly the same and overall is more expensive than a simple wedding need be.
I also used to work in the wedding industry and it's absolutely possible to have a lovely wedding without spending much at all. Indeed if you strip it to bare bones it can be done for less than £200 which is cheap when you consider all the protection it confers.
As a side note, if he's as I am guessing a self employed sole trader tradesmen then in the event of a split such men can very easily minimise or altogether avoid paying child maintenance. Major loophole there.
You need this sorted ASAP I'd say