Usually when the kid is around 3 years old, he/she can independently go to 主日學.
But also depends on the kid ~~ my younger son went to 主日學 when he was 2.5 years old (because he was accompanied by his elder brother!!)
Also for some churches, the parents can 陪小朋友上 主日學?? But I think if your baby is just around 15 months, it is quite hard for him to sit still and understand the instructions.
So if you really want your kid to go to 主日學, you may need to ask different churches for details....it's hard to tell in here.