Games

Once an account is set up on a switch your kids will not need to remember password to access it. From what I recall the only time you might have to recall the password is to add funds to the account to buy games on the shop.

There is a companion parental control app that allows you limit screen time or access hours and filter games by age rating. You’ll still have full access to the console through a quick passcode.

If they're that young, they definitely don't need their own Switch.

You only really need an account to get DLC, but I suppose it's necessary these days. If you only have one switch for the family than you can make that account yourself. The kids would not have to have their own online accounts until they want to pay for their own content. (As I recall, Nintendo requires additional verification steps for accounts for under 13s, anyway. I think they require a $1 fee just to "prove" an adult approves the account.).

And one thing I forgot is that if there are DLC/digital copies active on a primary switch, all accounts can use it. So you can install those and anyone can play. Then, if they ever get their own devices and let you log in and download all that content, they will be able to use it, subject to phone-home provisions. Unless they buy their own copies on their own accounts -- then they will be able to use the DLC without phoning home.