Thank you Philip and Rosie. I have been doing some more research today on Elizabeth Ann's daughter, Ada Marion Parker Hill (adopted by the Hill family?). She married Paul Dean Barr in Tulsa, Oklahoma. Her death record shows that she was born England on 18 March 1891 and died 12 July 1920, Tulsa, but her birth is also registered in Kings, New York as 21 December 1891.
I don't know how to check which one is correct. Her sister, Ada Ellen Parker was born 6 August 1890 in London, so Ada Marion would have been born exactly 9 months later which seems quite soon after Ada Ellen's birth. Ada Ellen Parker died June quarter 1891 but she is not on the 1891 census with her parents. Elizabeth Ann is only 17 at the time of the 1891 census but had already had 2 children (George Thomas and Ada Ellen), both of whom died in infancy. She married George Parker in January 1889 when she was only 15 (born 18 July 1873).
If Ada Marion Parker was born in England in March 1891, she would have accompanied her mother to the US in late 1891 or even 1892 (which is when Annie Clayton says she emigrated on the 1900 US census). If Ada Marion was born in Kings in December 1891, then Elizabeth Ann travelled to the US on her own.
George Parker, Elizabeth Ann's husband, emigrated in October 1893 on the Teutonic (departed Liverpool?)
In late 1890s, George divorced Elizabeth Ann (Annie), remarried, and had about 6 or 7 children with his second wife, Florence Schmitz.
There is no record that I can find of Annie Clayton (she reverted to her maiden name on the 1900 US census) after this census or of her son, George Parker born Kings, NY 26 October1894. I can only presume that he was fostered or adopted or was in care.