No it is not stupid. It matters not that there is no spiritual significance. This is wrong. Suppose your child came home dyed blue? Would that be OK with you?
Even though you think there is no religious intent, some people believe that a person's body is their holy temple not to be defiled in any way.
1 Corinthians 3:16 — Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you?1 Corinthians 3:17 — If any man defile the temple of God, him shall God destroy; for the temple of God is holy, which temple ye are.
On a more personal level, I cannot even stand to accidentally get an ink mark on my hand. I would be very upset if a teacher put a tattoo on my child. An impressionable child may be influenced to think it is acceptable to get a "tramp stamp" tattoo as a teenager.
It is also forbidden. Ye shall not make any cuttings in your flesh for the dead, nor print any marks upon you: I am the LORD. Lev 19:28