Yes - it is back to court you go -- your ex is in contempt of court by refusing to follow a court order...judges don't take kindly to being second guessed! Inform her, in writing by registered mail, of your intent to collect the child/ren for visitation with a set date/time and location for exchange -- when she refuses/fails to meet you for the exchange your next stop is the courthouse to file charges against her.
One thought - from reading your post it only states she is required to meet you halfway every other trip -- why not travel the entire distance to collect the children and then consider the "halfway trip" for her to get them back (she is very unlikely to fail to comply if it means the kids would be staying with you until she does)...it may leave you open for legal charges on her part, but technically you would not be in violation of the order - she would by refusing to meet you to get them back. If you truly want to see your children it should be worth the risk!
Good luck and God Bless
