In Umbraco 14 (I'm running version 14.2.0) you can select a document type, got to Structure, then rearrange the "Allowed child node types" to the desired order. I checked the cmsContentTypeAllowedContentType table and confirmed that the SortOrder column updates when I save. However, refreshing the page does not keep the items in the correct sort order and when adding content in the Create dialog, the "Create an item under..." list is also not sorted according to that sort order.