Is the contract considered void if the mother bought a house and paid for it, then asked to register the contract in the name of her children instead of her husband, who initially bought the house? And what is the religious ruling on that?

If the father purchased the house before the seller's death illness, and the contract is valid but not notarized, then the house belongs to the father. It is permissible for the father's wife to notarize the contract in the name of the children if it is done with her husband's permission. The father should be asked about his intention in doing so: Is it an immediate gift to the children, or ownership transferred after his death, excluding the rest of the heirs?