Interesting question.
You can try this syntax as a formula rich text field.
var text RID = If([Record ID#] > 0, ToText([Record ID#]), "%%rid%%");
var text AddChild = URLRoot() & "db/" & [_DBID_CHILD]
& "?act=API_GenAddRecordForm"
& "&_fid_7=" & $RID;
var text DisplayNewParent = URLRoot() & "db/" & [_DBID_PARENT] & "?a=dr&rid=";
var text URL =
$AddChild
& "&NextURL=" & URLEncode($DisplayNewParent) & $RID;
"<a class='SaveBeforeNavigating' data-replaceRid=true style=\"text-decoration:none; background: #4b7097; border-radius: 5px; color: #ffffff; display: inline-block; padding: 0px 0px 0px 0px; width:100px; text-align: center; text-shadow: none; border: 2px solid #030404; font-size: 12px \"href='"
& $URL
& "'>Save & Add Child</a>"
//NOTE: The %%RID%% does not seem to like being URLEncoded.
// note: it seems to be ignoring the NextURL as it seems to land me back on the parent record in edit mode as opposed to view mode, when I use the button in add or in edit modes.