Stair rail ballisters are too wide apart.

I moved into a house that had an open staircase to a lower level. The rails are about five inches apart or more in some places. I have small grandchildren and don't want them falling through it. They appear to be solidly attach. The rail is thicker at bottom and top than middle. Rails are white and posts and top rail are oak. Any suggestions besides removing the whole thing and starting over.
q stair rail ballisters are too wide apart
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