I just don't want to break her heart. There is that too. I feel really shitty about it, how could I be so careless?
Aww, I think I might just tell her exactly what happened. I don't think it was being careless, it's hard to keep stuff safe with little kids around because they get into things you may not even notice. Also, if your mom is anything like mine, grandkids can do no wrong. I don't think it's "blaming" your daughter if she legitimately knocked it into the trash. That's not something she did on purpose. And explaining that shows that it wasn't a matter of you just not caring about it, it legitimately got lost by mistake.
I'd also add "I feel really bad I didn't tell you sooner, but I kept hoping I was wrong and it would turn up. I'm so sorry".
It sounds like you generally have a good relationship with your mom, so I think if you're honest she's not going to assume you had ill intent here. She may be sad, but she'll get over it. Sometimes shit happens. I'm sure this isn't the first time she has lost something that had sentimental value.
I think this is a great way to deal with it. It's being completely honest while not dragging yourself through the mud. Your mom will understand.
A few years ago, my husband and I bought a really special piece while in Paris. I wore it on a trip a few years later, and decided it would be safer if I took it off. I put it safely away in my crappy makeup bag. A few months later, I bought myself a new bag and threw the other one in the garbage chute after giving it a cursory clean out. Yep. My very special necklace was still inside. I felt literally sick about it, but I told him right away. It wasn't insured or anything. Heartbreaking. But despite our shared sentimental connection, we both understood it's just a possession.
If it was my mum I would keep lying and tell her it's in the safe.
But that's because she would not react like a reasonable human being if I told her I'd lost it and instead act like a martyr with an ungrateful careless daughter until the end of times.