Giftover to grandchildren
Section 33N of the Succession Act 1981 ensures that if you leave part of your estate to one of your children and that child predeceases you, then the share of your estate which you left to your now deceased child, will be divided equally amongst your deceased child's children (your grandchildren) upon your death.
This is how the law says that your Will works even if you Will does not say so and to change this, if you do not want the giftover to grandchildren to apply, then you must specifically say so in your Will.
Also, you need to be aware in these days of blended families, that the giftover to grandchildren does not apply in the case of a stepchild, unless you specifically state in your Will that this is what you want to happen.
It is always best to get an experienced solicitor to prepare your Will.