Inheritance is inherently unfair, so I think taking into account what one person might inherit from someone else is pointless and will make them feel less a part of your family. Split it equally between all your and your husband's children.
I completely agree with this. Whatever promises you and your DH make to each other, and if the survivor faithfully carries out your joint plans to the letter, consider the following possibility:
Your DSS's mum has since married again, to a man with children of his own. She dies the day after the second of you two goes and the very next day, her husband is off to the solicitor to make sure that all of what is now his money (including all of what originally belonged to your DSS's mum - his ex-wife, even if she'd been earning £1m a year and he'd been on £15K) goes to his children and not a penny to his own step-son. What if he says "I married her, not her son - he isn't my responsibility, so he isn't getting a penny of my money"? Morally reprehensible but perfectly legally valid. He could even go one step further and say "Well, he had his own dad and inherited from him".
Yes, an inheritance is pragmatically financial, but that's by-the-by because the entire reason for leaving it to somebody is that you love them. You left it to your kids, because you love them; you didn't leave it to your milkman, because you have a friendly arrangement but you don't love him (unless your milkman is also your husband, son or grandson, I suppose!).
It's the splitting of your OWN worldly goods and assets that matters and not what financial position your heirs independently end up in. They could be millionaires or low-earners, marry millionaires or low-earners, win the lottery, lose their house when it burns down and they were too poor to afford home insurance, lose the use of their limbs in a car accident and have to give up their £200K job etc. etc. You can't legislate outside of your own circumstances.
Personally, I think the only justifiable reason for varying this principle is if one of your children is very disabled and will need expensive care, not be able to work to buy their own home etc. whilst your other children are fit and healthy; but even then, one of your healthy children could have a life-changing accident the day after they inherit whilst your disabled child could win £100m on the Euromillions.