Yes, you can still file a 2018 tax return, but only on paper; you cannot e-file. You cannot use TurboTax online, but can purchase download software* (or just get paper forms from the IRS**).
You report the income from your own records. The IRS considers undocumented cash income (no W-2 or 1099-MISC), for work performed, to be self employment income. Enter at "Business Income & Expenses" and TurboTax (TT) will complete Schedule C or C-EZ for you and allow you to deduct any expenses associated with this income. You'll also have to pay self employment tax (social security & medicare tax-about 14%) on any profits greater than $400. You do not report the names of people who paid you, just the total amount.
A child can be the “qualifying child” dependent of any close relative in the household. If you live with someone else, e.g. your parents, be sure they (or the other parent) didn't already claim the child for 2018.
*Prepare past year returns with TT https://turbotax.intuit.com/personal-taxes/past-years-products/
**IRS past yer forms https://www.irs.gov/forms-pubs/prior-year
