There are two tombs.
One is an island in which Edmund has lived for almost all his life.
But that is just his memorial. As he lived on that island for a very long time, it is the place where most of his memories are.
So, it felt appropriate to build a tomb for him there.
His valuables were buried to remember and honor his memories. Starting from the day he was born, all the things he used or cherished were buried there.
It was one of the hardest decisions Anna took in her life. Although she wanted to keep them to remember her son, she knew that every time she saw something that belonged to him, she would break down.
So, with everyone's insistence and suggestion, she made her heart a rock and buried everything that belonged to him on that island and decided to visit that place only on her son's birthday which is unfortunately the same as his death day.