I haven't filed in 9 years. I have had two kids since. How far back can I do taxes and can I claim my kids?

Wow---have you had taxable income in those nine years?  You can only file and receive a tax refund for as far back as 2016.  Child-related credits are based on having income earned from working.  Did you work and earn income during those years?  Do you have W-2's, 1099's etc. to use on the returns?

The current online program can only be used for a 2019 return.   You cannot enter income from any other year into the 2019 program.

To file a return for a prior tax year

If you need to prepare a return for 2016, 2017, or 2018 you can purchase and download desktop software to do it, then print, sign,  and mail the return(s)

https://turbotax.intuit.com/personal-taxes/past-years-products/

Remember to prepare your state return as well—if you live in a state that has a state income tax.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Note:  The desktop software you need to prepare the prior year return must be installed/downloaded to a full PC or Mac.  It cannot be used on a mobile device.   Each tax year requires the use of the specific software for the year.

If you are getting a refund, there is not a penalty for filing past the deadline.  If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.

Each tax year must be mailed in its own envelope.  Do not combine them.

https://ttlc.intuit.com/questions/1900990-how-do-i-file-a-prior-year-tax-return

If you have had taxable income during the past nine years and have not been filing tax returns, you should really seek help from a paid tax pro (not a seasonal storefront tax place) who could help you sort out what may be a real puzzle.