Every passports need their own visa. I applied for my young kids dtv (paperwork is easier than main applicant).
Yes, some people choose not to get a long term visa for their kids as there are no fines etc however, they will be overstaying their tourist visa which may cause issues when they're adults applying for visas to other countries when asked "have you ever overstayed on a visa". Something to consider as the children might grow up and not know this, if their parents don't inform them