Every studio has a different "vibe" and every place is different in terms of what it expects, but no one expects your child to walk in knowing the formalities. He'll pick them up from instruction and from emulating the other students.
Some things to do before you go in:
-Check the Better Business Bureau (if you're in the U.S. or Canada) and see if there have been any complaints or negative history. Money-related complaints can indicate that there is some sharp dealing going on with their contracts and fees.
-Search for online reviews from places like yahoo and google reviews, as well as bullshido.net.
-Look at the school's website. Here you can read instructor bios, look at class schedules, and sometimes look over preliminary and belt-testing fees. You can sometimes schedule a trial class or an appointment to take a tour through the website.
-Find out about the major organizations in the martial art you're looking at. The instructors should have a certificate or diploma from one or more of these organizations.
To ask when when you're there:
-What organizations do the instructors belong to? What is their lineage? Here "lineage" means teaching lineage. Most instructors will be able to tell you who trained them and who trained their instructor.
-What are the fees involved/how long is the contract? People are usually reluctant to tell you all of their fees, but if you press for the information you can get it. Make sure they tell you about any joining fees, monthly fees, belt testing fees, equipment fees, locker fees, etc. so you can add it all up. They may try to skirt around this question and just tell you something about the monthly/joining fee and omit the others. (This is pretty common-I'm not sure why.) Take into account that you may need extras down the road, like mouth guards and gloves.
-What kind of activities will my child be doing? How much of an emphasis is there on mindfulness, physical fitness and nutrition? I was a member of a dojo where, in the karate and taekwondo classes, there was an emphasis on mindfulness, awareness, kindness, and morality. Many dojos and martial arts don't put a heavy emphasis on the mindfulness side of things (Krav Maga involves hardly any) but some do. It depends on both the studio and the martial art you choose. For the kids at my dojo, there was also a heavy emphasis on self-defense (as opposed to just forms) and they would participate in kidnapping drills, sparring, and some punching/kicking of the bags. They also had "nutrition days" where they would learn about healthy foods as well as martial arts.
-Don't get sucked into a very long contract. A 1-2 year contract is usually too long, and most places let you pay by the month, with a discount if you do decide to sign a contract. I just signed a 6-month contract and I got 10% off of the fee. I signed it because I knew myself and knew that I wanted to do it for another six months. I wouldn't sign up for the long haul until you're sure about it and you've taken one or two trial classes.
-Watch for outrageous fees. Some places try to scalp you with joining fees, testing fees, and belt fees. Even at the highest rank possible, the belt testing fee shouldn't be more than $300 or so. (I paid $35 for my level testing fees, but it differs between arts/schools.)
-If they promise you or your child a black belt up front, be wary. A place that teaches a watered-down form of a martial art is called a "black belt factory" or a "McDojo". If they're promising you or your kid a black belt in 2 or 3 years, the place isn't legitimate. It may be good exercise and may increase your attention span or help with your posture, but it won't teach you good sparring or self-defense skills.
-Be wary of people who claim to have invented an "all-new" style of anything (like "Rex Kwon Do" from Napoleon Dynamite) or who aren't certified by a major organization. Also be wary of people who are only certified in the organization that their instructor made up himself/herself (Such as "Master Bob's Fighting Association) or that has very few members.