To answer my own question...
TreeNode document = treeProvider.SelectSingleDocument(documentId, true);
NodeSelectionParameters childrenSelectParams = new NodeSelectionParameters();
childrenSelectParams.AliasPath = string.Format("{0}/%", document.NodeAliasPath);
childrenSelectParams.MaxRelativeLevel = 1;
TreeNodeDataSet children = treeProvider.SelectNodes(childrenSelectParams);