There are four different things going on here.
First, the OP's father has died, intestate. A grant of administration intestate will need to be extracted to his estate. The work involved in this will depend on the assets, the costs will depend on the value to a certain extent, ie probate stamp duty is scaled on the value.
Secondly the children wish to disclaim their interest in the estate. They will need independent legal advice before they can do so, and that advice will have to include advice on the value of their entitlement.
Thirdly once the grant is extracted, an application will have to be made to transmit the title.
Finally OP's mother is also making a will,, this should be done immediately.
Each of these involves layers of complexity, but as a starting off point, the solicitor must be made aware of all assets and their values. Then she can quote.