Whether your kids are dependents and whether they should file a separate tax return from yours are two separate questions.
Regarding whether they're dependents, there are two types of dependents:
qualifying child and qualifying relative.
Based on the information provided I don't think they count as qualifying child dependents. For that they need to be either under age 19 at the end of 2023, under age 24 at the end of 2023 and
full-time students for at least five months during 2023, or any age and "permanently and totally disabled" (unable to do any sort of paid work due to a disability, with a condition expected by a doctor to last at least a year).
To be a qualifying relative dependent they need to be a relative, not a qualifying child dependent of anyone, with gross income less than $4,700, and for whom you provided over half of the person's support. Sounds like they didn't provide much of their own support if they were living with you and didn't have jobs, but if their investment income was over $4,700 then they aren't qualifying relative dependents either.
Now, for separate tax returns...it's improper for you to just take their income and report it as your own. There is
a form you can file as part of your return to elect to report your child's interest/dividend income on your own return, but just like for qualifying child dependency, you can only use that for kids in the 19-23 age group when they're full-time students.
Your kids should therefore probably be filing that income on their own return, if they're
even required to file one in the first place. Looks like the requirement to file starts at $13,850 of gross income for non-dependents and $1,250 of unearned investment income for dependents.