Are the children sinful for refusing to give their deceased father’s wife household items belonging to their mother, knowing that the wife is demanding them based on a list of household items signed by the father, while he did not buy her anything from those items except for a bedroom set?

The list of movables is your stepmother's right according to the prevailing custom in Egypt. Refusing to hand them over is an act of injustice and sin. It is advisable to settle the dispute with her, for reconciliation is best, as the Messenger of Allah, peace and blessings be upon him, said: "Reconciling people is better than the degree of fasting, prayer, and charity."