There are lots of really good answers on this thread.
If I might give the definitive answer: You need to take advice on your current Will since it may or may not provide for a surviving child to inherit your entire estate. It will be easy to deal with by way of a codicil.
Broadly speaking, s33 Wills Act 1837 says that if you leave a bequest to a child or issue (that's any descendant) and that child/issue dies before you, then their children inherit, UNLESS A CONTRARY INTENTION APPEARS IN THE WILL. I do not know what your current Will says, but the best way to ensure that the survivor inherits is to say "I declare that s33 Wills Act 1837 shall not apply to this my will".
How your will is drafted at present is important and you should ensure that you seek advice on it.