The article says he fell in. Some churches, those that believe in full emersion baptism, have a very large, deep baptismal font, sometimes built into the architecture of the building. I’ve seen them before with a glass wall, elevated in behind the pulpit in certain types of churches, several feet deep and at least six feet across. Obviously, this one was not in full view. The teenage girl charged with minding the toddler became distracted, the toddler wandered off, found the baptismal font, fell in and drowned before the teenage girl noticed that he’d gotten out of sight.
I remember our Baptismal font when I was a kid, there was at least one locked door to reach it and then another door to get to the water. Also the tank was empty when not in use.