There isn't a single income limit for all cases. In general, if you can claim the child as a dependent and the child is not blind, he or she is required to file for 2020 if any one of the following is true.
1. The child's unearned income was more than $1,100.
2. The child's earned income was more than $12,400.
3. The child's gross income was more than the larger of:
a. $1,100, or
b. The child's earned income (up to $12,050) plus $350.
You didn't say what kind of 1099 the child received. There are 21 different kinds. There is no plain Form 1099. All 1099 forms have one or more letters after the 1099. The most common are 1099-INT, 1099-DIV, 1099-B, 1099-NEC, 1099-R, and 1099-G. If it's a 1099-NEC and the amount is $400 or more, the child is required to file regardless of the requirements listed above. If it's a 1099-G or 1099-MISC, which box is the income in?
You can use the calculator at the following link on the IRS web site to see if your child is required to file a tax return. Remember that you are answering the questions as the child.
If the child is required to file, there are limited situations in which a child's investment income can be reported on the parent's tax return. However, that can result in paying more tax. In most situations the child's income has to be reported on the child's own tax return, and it's advisable to do that in almost all cases.