That's true! That said, the r-g-d.ru site I mentioned in the first post lists the same children as Wikipedia, with no mention of a Sophia.
It's difficult to follow internet trails without Russian as a language. That said, I've now found a site which lists both Sophia and Olga as daughters, plus a third daughter. In translation;
IGNATYEV Alexei Pavlovich 1842-1906
General of Cavalry. В 1885-1889
In 1885-1889 Irkutsk
in 1889-1896 Kiev governor-general. Killed SR.
Wife 1850-1944. SOFIA SERGEEVNA 1850-1944.
Children:
1. IGNATYEV, Alexey A. 1877-1954
Graf. The diplomat, Lt. General (1943), writer. In 1908-17 military attache
in Scandinavian countries, France, Major-General (1917). After October
Revolution on the side of Soviet power. Since 1927 the Soviet trade delegation
Paris. Since 1937 the Soviet Army. Memories of "Fifty years in service"
2. Pavel, 1878-1930.
He served in the Life Guards Hussars, a member of the First World War. In
1915 was sent to Paris for a Russian intelligence service. Later
head of the Russian mission to the inter-Allied Bureau at the Ministry of War
France. He was buried in the cemetery of Sainte-Genevieve-des-Bois.
3. SOFIA 1880-1935, husband Nikolai Zvegintsev,
4. Maria, died in maldenchestve (? - original is малденчестве),
5. SERGEY, 1888-1965, died in exile, was buried in the cemetery of Sainte-Genevieve-des-Bois. Сын от брака с актрисой Е.Н.РОЩИНОЙ-ИНСАРОВОЙ. Son from his marriage to actress E.N.ROSCHINOY-Insarov.
6. OLGA 1896-1949, after 1917 in exile after the war of 1941-1945 returned to the USSR.
site is
http://baza.vgd.ru/1/12797/10.htmSo it's number 6 on that list that I'm after!