With respect, you don’t sound very financially aware; certainly not enough to act as a trustee for your children.
Personally, I’d be doing the following:
Rip up your will and make a new one, with your brother and sister as trustees (assuming you trust them, at least more than your partner). Leave all your goods to your children, in trust.
Check your life insurance / pension etc, if you have any. Who are the beneficiaries? If not your kids, you can often make an expression of wish.
Ensure that this new savings account for your kids is safely locked down. If you don’t trust yourself not to access the account in the future (and tbh, I wouldn’t trust you), then perhaps your brother should be the one to open and manage the account going forward.
This may be your partner, and your children’s father, but he’s not a good one, and will spaff away any inheritance you give him. Protect your children’s financial health, if nothing else.