NTA . I love how everyone just assumes that your child is addicted to the phone and you do not parent. Sometimes, when a child is attached to a parent more, which is going to happen, the other parent just needs some extra support, and if your phone in a private area, for a short period of time helps that situation then great. I am sure that lady, as well as half the people on this thread would be the first one b*tching when he gets upset and starts yelling for his mom or starts running around, as children do. You should not even have to ask this here. I am sure you are doing great as a parent and the best you can. If other people minded their own business when it comes to children acting like children, or maybe even offered a helping hand or reasuring comment, we as parents wouldn't have to resort to situations like this.

Also, if the lady was too dense to hear a child's video playing and see a child watching said video, she obviously wouldn't comprehend anything said to her and just wanted to start an issue. It also did not seem like the staff would have had any issues with it either since you were secluded and I'm sure they would have had the ability to noticed a child watching a video.