If the children are yours then no visa needed, just British Passports.
If the children are not yours you may need to provide documentation that the biological father agrees to the children leaving.
If they are stepchildren, then as a first step please look at the UKBA webpage Bringing your children
It states:-
"If you are applying for a visa to come to the UK as a husband, wife or civil partner, and you want to bring your children with you, you must name them on your visa application. However, they will also need to apply for their own visas."
Just follow the links including the Immigration Rules, part 8 link on the right-hand side.
Hope this helps.
