Voluntary disclosure of tax fraud .. how harsh a punishment

I’d wanted to originally do an amended return but need to be fully honest. Here’s my story on what happened. I claimed my sisters kids because they moved in with me. She was okay with me claiming them since she was in another state. The daycare they attended gave me a tax form and ADMITTED they made the form appear more was paid than actually had been. Lie number 1. Number two , although they lived with me .. my sister paid the daycare fee. I never did. I still received the credit although she paid. Lastly I didn’t know then that the credit could only be claimed by them attending daycare as a means for me to be able to work. They went more so for leisure .. I worked overnight so it was no benefit for me. I didn’t know back then (years ago) that I could claim the kids without claiming the daycare credit. I have NO clue how much was actually paid to the daycare since the owner lied on the form. I want to pay back the money I received for the childcare credit. I THINK the form said about $600 was paid? How harsh do you think the irs will be on me for filing a voluntary disclosure ?