"Things You Wouldn't Know If We Didn't Blog Intermittently."
If it's also Russian, it could even be fish eggs (caviar) (there seems to be some color leakage), but more likely peppercorns.
Caviar was also my first thought. But after thinking about how I'd go about getting it to stay in place I ruled that out. Peppercorns is probably the best guess based on what I can make out in the photo. But I would opt for cloves if I try doing this for my grandkids. Cloves have that little stem which would keep them securely anchored.
Cloves for eyes?
The reason I didn't say cloves is that they aren't perfectly round, but have the 4 spiky bits around the center sphere. Also, they're brown, not black. But Russian cloves might be different than what I'm used to.
Sorta looks like one of the little dears has lost an eye.
Peppercorn would go good as a flavor with eggs and radish as well as look right.
Looks like the original photo comes from here, and the recipe specifies peppercorns.https://www.keyingredient.com/recipes/2909585032/mice-and-cheese-appetizer/
Thank you, Cobwebs, for that source information. I've added it to the body of the post.