I do have a papper signed by there father saying that I do have full & soul custody of the girls and that I do not need his permission as far as he concerned, this was signed at a Notary's office well that be enough ??
Coming in to the UK for simply a visit, I shouldn't think that you would need this document from your children's father. As others have said, if immigration asks you why you are here, just say it's for a vacation. They'll most likely smile and nod you through.
I've found that dealing with immigration, the less information you volunteer, the better off you are.
However, I did have a noterised document regarding custody when I applied for my fiance visa. If I remember correctly, you do have to demonstrate that you have the agreement of the children's father to bring them to the United Kingdom to live permanently.