I also prefer the self-checkout because the line-ups are shorter and like Larynxa I whip through it . However I’ll use the cashier ones when I’m too lazy to bag.
The grocery store machine I use is not loud but I totally hate the Walmart ones. They are so damn loud it’s really annoying. Maybe they have a lot of old hearing impaired people who go there? On one occasion where my debit card was overdrawn and I had to switch cards it practically shouted to the next county! YOU HAVE INSUFFICIENT FUNDS, DID YOU HEAAARRRR ME EVERYBODY IN STORE INSUFFICIENT FUNDS….. !!!!!!
I’m surprised they don’t have a red light and a siren to go with it LOL!
As far as kids and checking out – I can’t stand it either but since I’m so impatient I decided to be one of those nice patient cheery moms for a change and not constantly yell at them to hurry up so I would let them help. They weren’t really that young at the time – early high school age and they scanned while I bagged so in essence it went quickly.. But I ended up yelling at them anyway if they stated to look for a bar code because they can be really slow at stuff. I was always worried that people behind me were as impatient as I am so I was always rushing them. They were only trying to be helpful so I probably I scarred them for life!
Actually, I never let them help shop either which might explain some of those ditzy cashiers you all are complaining about 
The older one has no clue about grocery stuff and got a job as a cashier. ( Go figure!) They gave her one day of training with another cashier and then expected her to work alone and memorize all those flippin’ codes on top of it. She was not particularly gifted shall we say in this skill so got fired pretty quickly. I suspect she has a lot of my add traits which is why she didn’t bother to go around the store and “study” as it were.
So the next time you see one of those patient annoying mothers in the grocery store letting her kids help it might be the making of an efficient future cashier They won’t have to ask the customer if the green thing they are buying is an avocado or a kiwi