Princess Margaretha and Prince Nicolaus had a fourth child as well. Their first child died very soon after his birth. I found on a Swedish site a Liechtensteiner family tree with a mention of little Leopold.
http://w1.182.telia.com/~u18207700/kungalistaL.htm
LEOPOLD AV LIECHTENSTEIN 1984 - 1984
Äldste son till prinsessan MARGARETHA AV LUXEMBURG 1957- och hennes man prins NIKOLAUS AV LIECHTENSTEIN 1947-
PRINS AV LIECHTENSTEIN
FÖDD: 20 maj 1984 i Brüssel, Belgien
NAMN: LEOPOLD Emanuel
DÖPT:
SYSKON: Maria Annunciata, Marie Astrid, Josef-Emanuel
DÖD: 22 maj 1984 i Brüssel, Belgien
Swedish is not my mother tonque, but I tried to translate it in English (which - you can see it - is neither my mother tongue... ).
Leopold of Liechtenstein 1984-1984
The oldest son of princess MARGARETHA of LUXEMBURG 1957 – and his husband prince NIKOLAUS of LIECHTENSTEIN 1947-
PRINCE OF LIECHTENSTEIN
Born: 20 May 1984 in Brüssel, Belgien
Name: LEOPOLD Emanuel
Christened:
Siblings: Maria Annunciata, Marie Astrid, Josef-Emanuel
Dead: 22 maj 1984 i Brüssel, Belgien