The base design of the sitemap provider is supposed to allow for optimal retrieval of nodes. This means that if you request a particular node, the provider should retreive that node and only that node. But this can get tricky since the node has references to it's parent and child nodes The solution is to lazily retreive the parent/child nodes "on demand". This means that until you actually request those properties, they aren't populated. Now, there are ways to prefetch these if you know ahead of time but that's a different track.
One of the ways we've dealt with potential issues due to the tight coupling of these two objects (SiteMapProvider and SiteMapNode) is to always call into BuildSiteMap when these properties are accessed to ensure the provider is initialized and up to date. So, you can see then the recursion: BuildSiteMap requests ChildNodes which calls into BuildSiteMap.

The solution is to store the child/parent relationships in a separate look up mechanism (like a table) and reference it from there. I understand that it makes it much more complicated to work with this but that is part of the reason why the StaticSiteMapProvider and it's AddNode/RemoveNode/Clear methods are availible."