My wife and I are very happily married in our mid-fifties.
We each have children from our first marriages, all over 21.

At present we have mirror wills leaving everything to the other unless the
other is dead or dies within 28 days in which case it is shared equally
between the children with any already deceased childs share split equally
between their children.

It is not inconceivable that one of us might survive the other by 30 or 40
years and possible marry again.

How can we ensure that the interests of the children of the first deceased
are protected? We have no reason to distrust each other but anything could
happen in the possible timescale available.
Total net assets are around GBP400K including a house worth about GBP350K.

Probably the only way to be sure would involve setting up a trust of some
kind. However, this can be complex and expensive not to mention the fact
that governments have a nasty habit of changing the rules. Also remarriage
means that a will made before the marriage is no longer valid. There is an
exception which means a will specifically made "in contemplation of marriage
to a specific person" remains valid if the marriage takes place. It is not a
job that can be DIYed without huge risk that the consequences will not be
what you want to happen.

Peter Crosland