I don't even do fake nails because they get in my way, I can't imagine a 5 year old finds them all that convenient.

I'd worry more though if it turned into a regular thing, maybe it is just really some sort of special treat, though I still find it kind of age inappropriate.
I wouldn't want my child getting a real tattoo either because of the fact that young people change their minds so quick on things sometimes. What is cool to them at 12 may be the worst thing in the world at 13. Mine sports temp tattoos often though and he is 5, I've often wondered what a terrible mom people may consider me for that

, but to me it is just temporary and I think he just thinks their cool because DH and I both have real ones and is trying to imitate us.