I have a kid. He's just 5

OMG. He's a bullet hell cave fan (mister in a chewlix arcade set up) and wants/needs dad's help. I have barely touched a shmup or a shooter let alone a BH game since I was kid. Where does a dad start to learn? We were Mario Maker 2 fans just 6 months ago wtf happened. Sincerely a fgc dad. This is not what I expected.

I'm not sure there's any silver bullet (heh) to learning Cave-style bullet hell games other than just practice. Some general tips to help you get started learning one, though:

• Familiarizing yourself with the stages and bosses is the first thing. Don't worry about how well you do at first, just play the game a bunch (continuing over and over until you reach the end) to get exposure to everything and build familiarity.
• Use all of your resources. Usually this means bombs -- if you think you might die in a certain situation, use a bomb if you have one to get out of jail free. Try to never die with bombs still in stock. This reflex alone will get you remarkably far.
• Try to minimize your movements, don't fly around wildly with no plan. A lot of bullets in bullet-hell games are loosely aimed in your direction, so a good general approach can be to slowly inch your way across the bottom of the screen from one side to the other, then finding a good opportunity to "cut back" and reverse direction to go back the other way.
• There are sometimes multiple ways to solve a problem. If you can't survive a certain bullet pattern, can you dodge around the whole pattern (macro-dodging)? Can you dodge inside it (micro-dodging)? Can you just speed-kill the enemy outright before it can shoot all those bullets? If the bullets are aimed at you, can you position yourself on the screen so that the enemy fires them off-screen or in some safer direction so that you can more easily dodge around them?
• In most Cave games, holding the fire button will give you a more powerful focused shot or "laser," which will usually also lower your movement speed. This is great for helping you make precise dodges or for doing more damage to large enemies or bosses. Tapping the button will give you your regular shot, which is useful when you're facing lots of smaller enemies, or if you need to move faster. Most Cave games also allow you to enable a separate "rapid shot" or "autofire" button which is just a convenience shortcut for tapping the main fire button over and over -- strongly consider turning this on, especially if you have any concerns about RSI. (Since it sounds like you're playing arcade games, you'll need to go into the game's service menu to turn that on.)