Here are three short ways to say that, written for a 7-year-old:
- A glacier is a slow ice giant that sometimes drops big icebergs into the sea.
- Glaciers move slowly and sometimes send huge icebergs into the ocean.
- Big, slow glaciers sometimes break off giant icebergs into the sea.
How I made them simpler:
1) Kept "glacier" but called it "big/slow"; 2) used easy verbs like "move," "send," or "break off" instead of "drops"; 3) kept "icebergs" and "sea/ocean" so the meaning stays clear.