This last time my girls came over, they were stopped in passport control, and told that the next time they come over, they would need a letter saying that their dad knew they were traveling over, etc. They didn't mention if it needed to be notorized tho. Just to play it safe, if your ex will write a letter, it is better to send it on with your child. Mind you, this was the first time they had traveled over without me traveling with them, so that could have been why they were told they needed a letter.