Whether she is an LLC is irrelevant for her 2023 return. She has self-employment income.
If you lived with her for the whole year in 2023 and earned less than $4700 then you can be claimed as a dependent and she can claim the child as a dependent. If she is the child's bio-parent she can also file as Head of Household.
"my son and I are the dependents..."
If the child is your son, but not your GF's bio-child, then she cannot file as HOH or get any child-related credits like child tax credit for claiming him. She can only get the $500 credit for other dependents for claiming each of you. You and your son can only be claimed as dependents if you lived together for the entire year.
If you have self-employment income you will pay self-employment tax for Social Security and Medicare. You will need to use online Premium software or any version of the CD/download so that you can prepare a Schedule C for your business expenses.
https://ttlc.intuit.com/questions/2926899-how-does-my-side-job-affect-my-taxes
https://ttlc.intuit.com/community/self-employed/help/what-is-the-self-employment-tax/00/25922
https://ttlc.intuit.com/questions/2902389-why-am-i-paying-self-employment-tax
https://ttlc.intuit.com/questions/1901340-where-do-i-enter-schedule-c
https://ttlc.intuit.com/questions/3398950-what-self-employed-expenses-can-i-deduct
https://blog.turbotax.intuit.com/self-employed/self-employed-tax-deductions-
calculator-2021-2022-50907/
https://ttlc.intuit.com/questions/1901110-do-i-need-to-make-estimated-tax-payments-to-the-irs
https://turbotax.intuit.com/tax-tools/calculators/self-employed/