I want hitch hikers!

I have a nano build that is doing good but is too small for fish. Have some crabs and one soft coral. I'm trying to teach my boys about ecosystems and the amount of life a reef can support.... Well I'm trying to teach them about all the creepy crawlies but I can't find any! My LFSs have super clean rock that at best MIGHT have some copapods... Can anyone help us with this problem? My kids are losing interest fast