Well, it's a semi-choice. You don't have to claim dependents, but your dependents must say "Yes, I can be claimed as a dependent" if they can be claimed, even if you aren't going to claim them.
First, a child must always file a return in their own name to report income earned from working. (Sometimes if the child's only income is from investments, the income can be added to the parent's return.) The child will be asked "Can you be claimed as a dependent on someone else's tax return?" The child must answer "yes" if they can be claimed, even if the person who could claim them doesn't want to claim them. If the child answers "NO", that they can't be claimed, when they could be, that could be considered tax fraud.
There is a second question, "Will the person who could claim you, actually claim you this year?" This question only affects the child's eligibility for the American Opportunity Credit. In some cases (usually where the parent has a very high income) the child will get a larger AOTC than the parent, if the parent who could claim them, does not claim them.
For an 18 year child, if they live at home, they must answer "yes someone else can claim me", unless they provided more than half their total financial support.
For a 19 year old child, they will answer "yes, someone can claim me" if one of these two situations is true:
a. They are a full-time student, and don't provide more than half their own support.
b. You provide more than half their support and they have less than $4300 of taxable income.
If their income is too high and they are not a student, or they are a student but they pay more than half their own way, they can't be claimed as dependents.
As far as benefits,
For 2021 only, if you claim the children as dependents, you will get a $500 "other dependent" credit and a $1400 stimulus rebate for each child, unless you already got the $1400 stimulus check.
If the child answers "no one else can claim me as a dependent", they will get a $1400 stimulus rebate in their own name, even if you previously got a check for them. However, if they answer no just to get the $1400, that would be tax fraud, and the IRS has 6 years to audit them.
