So many concepts in the Bible are difficult for children to understand. All young children (and many older children) are concrete thinkers. When they read scriptures talking about God using a “refiner’s fire” (Zechariah 13:9 and more), they may believe God literally uses fire somehow. Even though the references to God as the potter and us as His clay (Isaiah 64:8) are a bit more concrete, they can still confuse children, who may wonder how they are clay.
Sometimes actually showing children items and explaining them in more depth can help them process metaphors and analogies in the Bible a bit more easily. When you think of all of those scriptures and others like them, there is a basic underlying concept. Because we – in this case your children – are born as babies into a fallen world, we aren’t fully yet what God created us to be.
God sends circumstances and people, gives us the Bible, and at baptism the Holy Spirit to help us grow to become more like His image in which we were created. What better example for that than sea glass?