Okay, Ruby/Dixon children:
Harry J. Ruby (mother's maiden name Dickson), 1912
Joseph C Ruby (mmn Dixon, died young), b. 1913
Charles Ruby (mmn Dixon, died young), b. 1915
and then your Frederick.
One of the birth certificates (say Frederick's) will confirm Ann/Annie's first name AND, if you're lucky, also list her previous married name.
There's a possible Annie Alice M. Dixon married in 1902, Poplar - possible spouse Albert Edwin Dodd. But I'm still not having any luck finding the Ruby/Rothstein (other possible name spellings?) marriage.
Does your friend remember when Elizabeth and Winnifred were born, or who they married?
Or when Isaac and Ann died?
!!!!!!
http://www.oldbaileyonline.org/browse.jsp?id=t19101011-name-110&div=t19101011-29#highlight - mentions Mrs Elizabeth Dixon, Annie Dodd her daughter, witnesses in the trial of Isaac
Rottstein, charged with manslaughter (found guilty, but under provocation, sentenced to two months).
There is a mention of him saying something about "his wife" being there to the police originally, although he was there with Elizabeth Dixon, Annie Dodd, and another woman (a daughter in law of Elizabeth). In court he amends it to 'a young lady, her mother and her sister in law' - and says that the young lady (Annie?) was heavily pregnant.
He says he was 23, and a tailor. This could be another mention of him, with his father! (This time, as witnesses for the prosecution):
http://www.oldbaileyonline.org/browse.jsp?id=t19061022-name-386&div=t19061022-66#highlight1901 shows an Isaac Rottstein, b. Bristol, aged 13, in Limehouse, with father Reuben, from Germany, and the rest of the family. He is on freebmd.rootsweb.com as Isaac Rothstein, b. Bristol.
I think Annie might also be in Limehouse, along with mother Elizabeth, 54, b. Devon - I can't see full details of the 1901, someone else should be able to get you the full censuses.
After Isaac was released, he probably changed his name to Ruby and set up house with Annie and her daughters in Stepney. They did not necessarily marry.
