Get the children's School and Medical Records, look for evidence of neglect and/or abuse. Alienated children frequently have high absenteeism, behavioral problems, and poor school performance due to the emotional and psychological abuse they suffer.
Read 'Tips for Getting Started'. Hire an attorney near the children, and make sure you REGISTER your out-of-state order (see UCCJA), if applicable, so that it can be enforced by the courts where your children live.
Above all, DOCUMENT! DOCUMENT! DOCUMENT!!!