Basically, becuase if the age the child's earnings will not matter when you file your 2019 taxes next year. If the minor has more than $1,050 of investment income such as interest and/or dividends, then the child is required to file a tax return. If they have more than $12,000 of earned income, then they are required to file a return.
Now if they have less than the above amounts they don't need to file a return at all. However, if any taxes were withheld on the earnings, then the only reason they would file a return would be to get any withheld taxes refunded to them.
As far as you claiming the minor child as a dependent, so long as they do not provide more than half of their own support, you claim them and it's not an issue. I don't know many 8 year olds that provide more than half their own support. 🙂
Overall the important things is, if/when the minor does complete their return, they must select the option for "I can be claimed on someone else's return." Otherwise, you will not be able to claim them as a dependent on your return.
