Are we entitled to force our deceased brother's wife to register a share of the house in her children's names, given that he had registered it in her name before he passed away, and she refuses to do so?

If your brother passed away and left behind only a wife, a son, and a daughter, then his estate is to be divided among them according to Islamic law. Registering the house in the wife's name is not considered a valid gift (hiba) under Islamic law unless the conditions for the enforceability of a gift are met. If it was a bequest (wasiyya) to the wife, it is invalid, because there is no bequest for an heir unless the other heirs permit it. These properties enter into the inheritance and are divided according to the legal shares. The matter can be referred to the judiciary to compel the wife to register the children's shares. However, if it was a valid gift with all its conditions met, such as possession (evacuating the house and handing it over to her), then you do not have the right to compel her to register the children's shares, because they have no right to it under Islamic law.