Should I speak up or hold my tongue (details inside)

I was just visiting with a nephew and his wife who have a two year old. On this and a previous visit, I’ve noticed to my consternation that they don’t always put her in her car seat. When they feel she is tired, the mother will sit in the back with the little girl on her lap. Always being in a car seat was one of my few cardinal rules when I was a parent of young kids and I feel this is really dangerous. If it were my son, I would definitely say something to him about my feelings. Should I to my nephew to whom I am quite close or should I butt out?