It is a misconception to feel that there is a title deed/document registered, it can not be challenged. Any such registered deed can be canceled through the process of court on such grounds stated by Shri Trivendra Sharma, by the right person at a right time.
To the specific question posed, though WILL can be challenged on other grounds, if ot is challenged on the signature of witnesses you can ask the children to give statement in court about the genuineness of the signature. Witnesses need not know the contents of the WILL. They are only authenticating the signature of the TESTATOR