The Wills Act provides a will is revoked by marriage or the commencing of a registered relationship unless the will is expressed to be made in contemplation of marriage or the registration of a relationship. If you die without a will or your will has been revoked you are intestate. If you enter a new relationship your new partner is entitled to administer your estate. If you do not have a new partner the children of your old marriage or relationship are entitled to administer your estate. If your children are under the age of 18 their parent can apply to administer your estate on your children's behalf.
Rule 44 of the Administration and Probate Rules enables your minor children's parent who is likely to be your former partner to control your estate. This is unlikely to be an outcome you would have intended. If you have been through an acrimonious property settlement with your former partner it is unlikely you would intend that former partner to have control of your estate for the benefit of your children upon your death.
The solution is simple; prepare a new Will and appoint a person you trust as your executor until your children are over the age of 18 when they are fully entitled to assume the responsibility of being both beneficiaries and executors of your estate.