In a suit affecting the parent-child relationship, the court may appoint joint managing conservators or may appoint a sole managing conservator. The managing conservator is that person with whom the children reside, who selects the home or domicile of the children and who has the responsibility for the normal day to day care of the children. This term means the custodian of the children or that person having custody of the children. If the parents are or will be separated, the court shall appoint at least one managing conservator. A managing conservator must be a parent, a competent adult, an authorized agency, or a licensed child-placing agency. A non-parent managing conservator can be a relative of the child or other competent adult. The best interest of the child is the foremost concern of the courts while making these decisions.