This happened to a friend of mine. The difference here is that she was the biological grandmother. You have no legal rights.
That said, file a police report. The police can call CPS or not. You do not need to call CPS because the children are NOT in danger. As a matter-of-fact, your ONLY concern is the best interest of the children. Your secondary concern are the parents; hence, file a police report.
Your next step is to file a motion for temporary guardianship, post-haste!
Document everything - and I mean everything. The behavior of the parents are suspicious and could cause harm to the child. As a matter of fact, in my opinion they already have - alienation of affection. This child has been bonded to the parent and a parent to immediately withdraw with no concern or contact is neglect.
Keep me informed. Your first call is to the police. Make a report and get a report #. You don't need to contact CPS - that's not your call if the child is safe. The police most likely won't if the house is in order and you inform them you are contacting the court.
Don't forget to get the officer's name and the report # and the dispatcher's name, date and time you called.
Start logging everything.