Did you and your husband not have some sort of "agreement" while he was still alive as to how this situation (ie one of you predeceasing the other) was to be handled?
It's often built into the wills of husband and wife by way of what is called a commorientes clause which explains what is to happen to their joint estate in the event that they die at the same time, or it can't be determined who died first - eg a car or plane accident.
My wife and I don't have children but we have several nieces and nephews each and our wills provide for an unequal distribution to them in the event of us dying at the same time.
My wife and i are agreed that if one of us predeceases the other, then the surviving spouse will leave their estate as per the existing commorientes clause.
Of course there is nothing preventing the surviving spouse breaking the "agreement", but if such an understanding existed then it seems to me fair and proper that the surviving spouse should honour it.
As others have said, it was also always open to your husband (and you) to protect the position of his (and your) respective children by ony leaving you a life interest - but it would appear he chose not to do it.
You are legally entitled to leave your estate however you like on your death, but if you'd agreed something with your husband prior to his death there is an argument to say you should be morally bound by it
[Edit - the reason my wife and I agreed to an unequal distribution between our respective nieces and nephews is because we each made/are making an unequal financial contribution to our marriage and we wanted our wills to reflect that. Any marriage with step-children on one side only needs to consider it too before one spouse dies and it's too late]