Is it permissible to seek divorce due to a lack of feeling safe with the husband and his constant threats, and would this be considered an injustice to the children?

Affinal kinship is a blessing from Allah and a sign of His power. The Prophet, peace and blessings be upon him, was an example in respecting his in-laws. What is happening between your husband and your mother is regrettable. They should be advised with wisdom, and reconciliation should be sought between them. Your husband is not permitted to harbor ill thoughts about your family, and his threats against you contradict the good companionship due to a wife. If you are severely harmed, you have the right to seek a divorce, but patience might be preferable, along with prayer and counsel. Divorce is not an injustice to children if done wisely. The husband should not prevent his wife from visiting her family. If he objects to frequent visits, then preserve this good, and come to an understanding with him to increase the visits.