How to deal with daughter who has buried a baby in my backyard and I did not know?

You don't "deal" with her. You call the police.

Damn. I mean, damn. That's a rough road there. It needs to be determined if it was a miscarriage or a live birth. If it was a live birth, you really do need to call the police. I know that's hard, but she can't be allowed to feel she could do something like that and not suffer the consequences.

You know now, so call the police immediately.  Don't allow this to be a family secret.  It will eat you alive and your daughter as well.  You deal with it the best way you know how, and that is to report it to the authorities.

You better call the police. Because if you dont, and the police gets to know of it later, even you would be held for conniving in the crime.

Bullshit question from posting bot.

Oh look, another post from paranoids anonymous.

Here's the thing. If you had knowledge of this and did NOT notify the authorities, you can be charged as an accessory after the fact and obstruction of justice. Both very serious charges.