If you live in a 'two-party' State, your ex must be informed that you are recording the conversation. You could also use an answering machine that continues to record even if the phone is picked up after it starts. If the message starts and the person talks, the court will reason that they knew they were being recorded (since that's what answering machines do).
As for making the child aware of what is going on, one suggestion is to have the tape recorder in plain sight when you answer the phone or when you call. Have your child with you when you press the record button, and make the note of time and day on the tape so he/she sees it. Your child won't be part of the decision of whether or not to record the call, but will be aware of it.