sadexpunk, you could say in the main provison, for illustration purposes only you understand, something like:
‘(1)I revoke all previous wills and testamentary dispositions
(2)If my wife [name] survives me I appoint her as my Executrix and leave the whole of my estate to her but if my wife has predeceased me then I appoint [name] as my Executor/trix and leave the whole of my estate to those of my children alive at my death in equal shares’
Depends on how old your children are and numerous other points (size and value of estate etc) and you would also need to make sure the will was properly executed and set out with your own details confirming it as your last will and testament etc.