It depends on where and what treatment. Keep in mind that some places like day spa's or places with lots of treatments and rooms could have relaxation massages and treatments in rooms close by and some people get upset when they hear children while paying big $$ to get pampered and relaxed. I say this because as a massage therapist it gets frustrating when parents bring unruly kids in and they are loud and i'm trying to work.
Having said that, lots of parents bring their kids in too if its a short appointment and thats fine, you just need to be repectful of other clients and therapists.
Maybe go somewhere like a hair dressers that also do beauty cause that is generally a 'louder' environment than a day spa etc

I also take ash to things like nail appoitments and hair appoitments etc