The assumptions at law have not quite caught up with the modern reality of blended families. If you have non-adopted stepchildren then you need to pay particular attention in your wills to make sure your wishes are carried out. If you want them to inherit something when you die you will have to set this out in the will. Stepchildren are not treated the same way as adopted children.
Likewise: if you marry someone who is not the parent of your children you should make a will and specify in your will that you want your children to inherit something when you die.
If you want your new spouse or civil partner to inherit everything when you die, and then have your combined assets divided between your children and their children you can either:
(a) discuss this with your spouse, make your wills and ensure that both sets of children are fairly dealt with on each will. If you want the different sets of children to inherit different proportions please follow our guidance; or
(b) get a bespoke pair of mutual wills drafted by a specialist solicitor.
Choosing (a) relies on trust and goodwill between you and your spouse or civil partner. (b) is likely to be more expensive and time consuming – but worth it if you believe it may be necessary to ensure fairness.