"I'm not sure if my kid is ready for lessons."
"I don't know anything about this - how do I know which instrument he'll like?"
I've written before that I don't think it matters that much which instrument you choose. Nor can I really be much help in that area - having never desired to play, say, the flute, I am utterly mystified at what would motivate someone to do that (and my mother is a flautist). Unfortunately, there is no personality test that will tell us which instrument will suit a particular person.
In an attempt to deal with some of this uncertainty, parents will request a trial lesson. However, the purpose of a trial lesson is not to see whether a child is ready for lessons or whether he will like the instrument. A one-time trial lesson is solely to determine whether you like the instructor. I guess some logistical aspects of the decision might be taken into account such as drive time, parking, noise level, and studio appearance, but these are secondary when there is a great connection with the teacher. Trial lessons don't provide much insight into readiness or interest.
What then, are you to do if you don't know whether your kid is ready for lessons? There are a few options.
When in doubt, wait. There is no harm in waiting. The critical period hypothesis for language and music, which suggests that children develop their aptitude before age six, does not mean that after age six learning language (and music) is hopeless. Rather, it implies that if children aren't exposed to language during the critical period, they're in trouble. Research is ongoing as to whether this applies equally to music, but chances are your kids have heard enough melody, harmony, and rhythm to have a lifetime of enjoyment ahead of them even if you wait until second or third grade or later to start them on formal lessons.
Group classes. Private lessons are not necessarily a big commitment - you pay a month or a semester at a time. However, it's a huge implied commitment: "We are going to spend the next seven years supporting our child as she develops mastery of the violin." There will be highs, lows, and plateaus. The relationship your child will have with her teacher is unusual - intense one-on-one time with an adult who's not a family member. In some cases, the teacher practically becomes a family member, which can present a challenge when it's time to move on.
If you're not quite ready for all that, I don't blame you. Group classes are a great alternative at any age: you can get a surprisingly in-depth experience at a fraction of the cost of private instruction. If you discover that the class isn't your cup of tea, you have fixed end points at which to bow out gracefully. Also, you can try a few different classes in order to explore different instruments and styles in a cost-effective way.
Short-term enrollment. If you're pretty sure that private lessons are what you want, you can do a trial for a specific amount of time. The important thing is that you decide ahead of time as a family what your commitment will be, rather than quitting when things get rough. This type of trial accommodates the student's needs, but not whims - it's a great way to hold a child accountable while still allowing for a change of direction.
Go all in. No matter what option you choose, do not hedge your bets. Fully expect your child to succeed, and you increase the chances of that happening. Assuming he'll quit after six months, likewise, will create a self-fulfilling prophecy. None of this, "we aren't renting a sax until he's sure he wants to do this!" The average ten-year-old is not equipped to weigh the implications of his decision. You make the call, and then give your child the support and encouragement he needs...and give yourself a little grace when it seems like the whole thing was a terrible idea.
The bottom line: You can't really go wrong, as long as you are being the grownup and owning the choice you make on behalf of your child.