Most kids join band in 6th grade. There are two major times when kids quit .... one at the end of the first year of middle school and one at the end of middle school. Our high school band director said that if he could get them on to the bus to their first football game, they were going to graduate high school still in band. Marching band takes a lot of time and effort but it's super social. Each section has a unique personality and the kids become "clarinets" or "flutes" or "trumpets" and it's amazing how the personalities of the kids start to fit their instrument.
If your kid has already tried violin and liked it, I would suspect your kid has a pretty good ear. Clarinet should be a good transition. If your kid tries it but discovers they would really prefer something else or to learn multiple instruments, let them try. We had lots of kids in our program that played different instruments in multiple bands.
As others have said, work with your band director to find a private teacher that your kid likes, or seek other clarinet parents and ask who they liked, so your kid can find a person they enjoy working with. Your kid may have a lesson once every one or two weeks. Music is supposed to be fun and enjoyable, not a death march, so if the kid hits a rough spot, let them have the experience and try not to pressure them.
Once your kid has an instrument, a band director, is in a band program, maybe has a private teacher and some band friends, everything else should just happen from there. You shouldn't have to provide much guidance.
Find out where your local music store is so that you can buy a box of reeds or get something fixed. If your kid sticks with clarinet and gets good, think about buying a reasonably good instrument for them sometime in high school. Kids start on rental instruments through a music store or the school, but if the kid really gets into it then your private teacher or band director can guide you to a better instrument once you are sure the kid wants to stick with it. Some instruments like tuba or percussion don't lend themselves to individually owned instruments.
Lots of bands go on trips. Send your kid if you can. If you go along as a chaperone, help out while staying away from your kid. They want you to be involved but magically disappear when they are hanging out with their friends.
Get involved with the band boosters. Even if you are not a musical person, you have some talent that the band boosters need, particularly in a marching band. Accounting, designing t-shirts, selling merch, building stuff, painting, driving trucks, unloading trucks, loading trucks, unloading trucks, loading trucks, unloading trucks, loading trucks, unloading trucks, serving pizza, making videos, whatever. They always need volunteers. Show up, do a job you know how to do or ask for one you don't, and keep a reasonable distance from your kid.