Let me start with this statement, almost all martial arts are good for kids. That being said you need to understand why your child wants to learn martial arts. Do they want to emulate what they see on TV, have a bully problem, lack focus or just want to try it because a friend is training? What excites them about martial arts?
Once you understand why the child wants to learn martial arts, then you can try to answer which is best for your child. To do this you also have a say right? Unless it's mainly because a friend is in a class that they want to join, you have an obligation to do a little research that goes beyond price of the classes.
Important points to ask are:
1) Does this school teach what my child specifically wants to learn? After all you don't want them to get bored 2 months in because it wasn't what they expected to do.
2) Does this school teach what I want my child to learn as well? Ask yourself what attributes you hope your child learns from the training. Fitness, self-defense, confidence, goal setting, achievement and respect are all good attributes to look for but ask how they teach these specifically. What is this school's focus for this class?
3) Does the schedule work for us? This shouldn't be the first concern but make sure you can arrange to attend all classes that you are paying for. Your family may have to change it's routine to accommodate. Location will play a part in scheduling so don't neglect travel time.
4) How much does it cost per month? Make sure you know what you are paying for up front. You are dealing with a business and you need to realize what this means to your wallet. Find out all of the additional fees you will be required to pay before you budget this activity. Most "large" schools will charge for registration fees, testing fees, uniform changes, equipment, association dues and sometimes more. You need to know what a monthly financial commitment this will be including these extra fees. Also, don't be afraid to ask why you are paying these fees. Usually, it is a way to make more money (remember it's a business) to pay for their overhead and not scare away potential customers with a high price up front. Don't get me wrong, throughout history students have paid for martial arts instruction, just make sure that is what you are paying for.
5) What is the commitment for my child to train at this school? Let's be honest, kids attention spans are fleeting at best. Will you be locked in to attend a specific number of classes or months with your membership? What is the cancellation policy? What is my risk assessment with this investment? How can I keep my child motivated? Really discuss any concerns you have before you sign up your child to a program.
I really hope this helps you in your journey to choose a martial arts program for your child. Above all else, if the child likes their training and you like the results, then you should always feel like you are getting more than you pay for.