Neither parent has custody, can one parent move the child out of state. And where would I start to file for custody of children

Probably best if you hire an attorney where the children have been living for 6 months and file a SAPCR (Suit to Affect the Parent Child Relationship.) You should be able to get Temporary Orders preventing the movement of the children. You need a court order in place.

You need an attorney to represent you and fight for the child custody. Inability to communicate with the parent is an important factor in establishing whether moving out of state is in best interest of the child.

Absent any court orders, either parent has the right to possession of the children. If you do not have orders in place, your first step is to hire an attorney to help you obtain orders; you will probably want a geographic restriction so that he is unable to move the children away from you. Texas courts typically want children to have the ability to see both parents, provided that the parents each provide a safe environment for the children.