Placing pebbles that are not made of stone?

I am taking my daughters to visit the Shoah memorial in Vienna tomorrow to see my great-grandfather and great-uncle’s names, and remember our history. The kids and I regained Austrian citizenship in 2021. My kids are now 6 and 8 so while we have talked broadly about the Shoah, they don’t know our entire family history.

This week we talked about how my grandfather and his two brothers being arrested and sent to Dachau, how my grandfather and one brother got out, and what happened to the remaining brother. We also talked about my great-grandfather being transported to Riga, but not actually making it there. We talked about the custom of placing a pebble, and the kids found a couple each to bring.

The issue is, they found some small water-worn ceramic pieces in the creek that are lovely, but not actually stones. Also, they wanted to leave a piece of sea glass they found this summer in Greece because it’s special to them. Would these be appropriate “pebbles” or no? Normally I’d let them just leave what has meaning to them, but the Shoah Memorial Wall is scrutinized more than, say, my grandfather’s grave in Minnesota, and I want to make sure we are respectful. Thank you for your help!