“Stop sexualising my daughter”

My cousin who is more like my sister we grew up in the same house has a 5 almost 6 year old daughter, they were at the beach yesterday, a very busy beach that was jam packed. I couldn’t go so she sent me photos and half way through looking at them they changed to photos of her daughter running around the beach stark naked, not an empty beach but the beach that in the background of the photos you could see at least 60+ people. Her daughter was playing in the water and running around, there are ones of her posing with her ice cream and others in the river but all still naked, no pants, vests etc. I replied asking what happened to her clothes and she said she just wanted to play in the river so they took them off and she run around like that for the rest of the day. I said that I didn’t find it overly appropriate given that she isn’t a baby or a toddler anymore and the beach was extremely busy, she could have at least put pants on her daughter. She got super mad and told me to stop sexualising her child. Now let’s be clear, this has nothing to do with finding a child’s body sexually appealing or unappealing, they are children, they are not sexual being to be looked at but I think at almost six years old running around stark naked in a public place with over 100 people there is heavily inappropriate. Even if it were all female or all children her age I would find it incredibly inappropriate. I told her that’s only my opinion and if she’s happy letting her run around naked then thats her decision she is the parent, I just personally wouldn’t. She hasn’t spoke to me since. She’s bitched a couple of times that I’m sexualising her child but other than that won’t talk to me. What do you think? Is it acceptable to let your child at that age run around naked? Is it only me that finds it incredibly inappropriate???