My 9-year old gets a transportation reimbursement from the state and he gets a 1099-NEC and a 1099-MISC for it. Do I need to file these in my return?

I think you're referring to the Kiddie Tax.

This only applies to unearned income.

The tax applies to dependent children under the age of 18 at the end of the tax year (or full-time students younger than 24) and works like this:

• The first $1,150 of unearned income is covered by the kiddie tax's standard deduction, so it isn't taxed.
• The next $1,150 is taxed at the child's marginal tax rate.
• Anything above $2,300 is taxed at the parents' marginal tax rate.
• If your child also has earned income, say from a summer job, the rules become more complicated. 
• To learn more, see IRS Publication 9291 or consult a tax advisor.

Topic No. 553 Tax on a Child's Investment and Other Unearned Income (Kiddie Tax)

This income is miscellaneous income, and your 9 year old gets the standard deduction od $12,950.  so they probably don't have to file a tax return.

Single dependents

 You must file a return if any of the following apply.

• Your unearned income was more than $1,150.
• Your earned income was more than $12,950.
• Your gross income was more than the larger of:
• $1,150, or
• Your earned income (up to $12,550) plus $400.

See Table 2:  Publication 501 (2022), Dependents, Standard Deduction, and Filing Information

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