Do not pool assets, or leave substantial assets to your partner in your will. You may have made wills dividing assets fairly between both families. The survivor may have every intention of honouring that will, but then a year after the first death an allurring siren/toyboy turns up, the survivor, quite uncharacteristically falls for them hook line and sinker, gives them money, gives them gifts and revises their will. The siren/toyboy gets all the money intended for the children.
If you keep control of your own assets, make a will, that may give a partner a life interest in them, but that they will then go to your children. You can then be sure that if your partner has a brainstorm after your death and goes off with the toyboy and gives him everything, the everything doesn't include your everything and your children are protected.