As soon as it becomes apparent certain clothes or even hairstyles are forced on people to show they are part of a group, a ban can be discussed. Especially when they are forced on young children.
Every fundamentalist will use the same argument as you do: that these children merely want to express their religious feelings. Sadly, you can't open that door to those that won't force it on their children, to protect those of those who will.
I'm sure a righteous god won't think wearing a uniform is all that important in showing your love.
Seeing where this comment thread originated: do you believe Trump supporters will stop hating trans people faster if we allow them to wear their religious uniforms?