Slate is between 2.5 and 4 on the Mohs scale ... almost as hard as marble and limestone, but not as hard as granite or natural quartz. I can't imagine a young child scribing that hard of stone; early teens, sure. But these are too well done; more on the level of artisan.

It would have to be a much harder stone tool applied with a great deal of pressure. That would require an adult allowing a child/young teen use of valuable tools. Unlikely.

Had these owls been exclusively in children's tombs one might consider it a toy. But they were in adults tombs as well. I think it would be an amulet or deity reference considering the characteristics of owls in general.