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).

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.