How do I add a 1099-Div form for a dependent child to the parent's return? If I just add it in the same way as the others, it doesn't let me say it belongs to a kid.

Do not enter it under the parent.

A separate tax return must be filed for children who have unearned income that is greater than $11,000 or any amount of earned income. If a child's unearned income is less than $11,000 and greater than $1,100, the child's unearned income can be included on their parent's income tax return.

 For the tax year 2021, the Kiddie Tax applies if your child has unearned income (usually from investments) exceeding $2,200, is required to file a return, isn’t filing jointly, and was age:

• 17 or younger at the end of 2021;
• 18 at the end of 2020, but only if their earned income (excluding scholarships in the case of a full-time student) didn’t exceed half of their support costs in 2020; or
• 19–23 at the end of 2020, but only if they were full-time students and their earned income (excluding scholarships) didn’t exceed half of their support costs in 2020.

 From <https://ttlc.intuit.com/community/income/help/what-is-the-kiddie-tax/00/25913>

Open or continue your return if you don't already have it open.

1. Income & Expenses
2. Scroll to Less Common Income
3. Select Child's Income (Under Age 24)
4. Continue through the interview