How old of kid are we talking about, and what physical size?
I’ve instructed a lot of kids into shooting sports and marksmanship in the last ~20+ years, and I made all of the mistakes everyone recommends people make as common advice - which is well meaning, but bad advice all the same.
My best advice is to get a 22Lr rifle and get an additional stock or an adjustable stock to fit their LOP, and stick a red dot on top. Do NOT get the “micro rifle” 22’s like Cricketts, Chipmunks, or Rascals, these are 100% wasted investment which kids will outgrow quickly, and guess what - a manual cocking single shot will absolutely NOT force a kid to learn trigger discipline. The instructor reinforced trigger discipline - not the action type. But also, these micro 22’s have super small and light form, but have lengths of pull which are still too long for the age of kids for which they’re commonly marketed. These typically have 11.5-11.75” LOP, which is typically suited for kids around 8-9yrs old, or older, whereas the action type is probably really only sensible for 3-5yr old kids.
Iron sights are fine, but they are the most challenging sighting method, so despite all of the Luddite “I learned on irons and I shoot fine” Fuddism, irons require more from the learner than necessary, and Red Dot sights reduce that burden. EVERYONE learns better when they can functionally segment fundamental skills and develop each through focused, independent effort, and removing the burden of sight alignment allows the kid to focus on target picture, breathing, and trigger control better to learn more quickly and develop deeper proficiency.
AR’s are very adaptable, but recognize, AR’s are very difficult to shoot well. So it’s a double edged sword.
Take-off stocks are very easy to find for a lot of models, and very cheap. Cutting these down to adapt LOP is very easy. Be mindful of comb height also - it’s very common for small faces to end up way too low under optics, so adding cheek risers is commonly needed.
We also have to acknowledge, if we buy a “youth rifle” with limited function or utility, then the kid will outgrow it quickly and the kid will lose interest and want something different - so the youth rifle becomes a sunk cost. And equally, the kid kinda loses connectivity with what was their first rifle - if you instead buy a more useful, conventional model, but adapt it for a small body, the kid can make use of the rifle throughout their entire life, which can carry considerable meaning in itself.
Applying all of what I learned in offering instruction for kids all of these years to my own son’s marksmanship journey: he started shooting Nerf blasters while he was 1yr old. He started shooting BB guns while he was 2yrs old, and 22LR later that year - then got his first 22LR rifles when he was ~2.5. He started shooting shotgun at 5, and started shooting an AR at 4. He shot a half mile for the first time when he was 6, and 1000yrds for the first time at 7, using my 6 Creedmoor and 6 Dasher rifles. He shot his first PRS match last season while he was 8yrs old. He started on a 10/22 with a modified stock and a combo red dot and laser sight, so I could see where he was aiming on target when breaking shots. When I bought his own 22’s for him, I got him a Marlin 60 with a red dot (Burris FastFire) and a Savage Mark II with a 3-9x scope. He started shooting that Savage Mark II out to 300yrds when he was 6. I got him a Ruger Charger 10/22 pistol when he was 7 or 8, which he has also shot out to 300yrds. We’ll finish up his own centerfire match rifle this summer when my new action delivers, and I’ll put my current Defiance Deviant into his Manners Compact Gen 2 stock, so he’ll have his own 6 Dasher. Dude is hugely capable behind a rifle, and it’s largely driven by the progressive learning process we put in front of him, including the rifles we’ve built and rebuilt to fit his body and skills at his iterative growth levels, and the strategic instruction and practice paradigm he’s been in to develop and sustain his skills.
Putting the kid behind a poor fitting stock with a rifle too big and heavy and too much recoil and an overly challenging sighting system just doesn’t make sense.