I have had another look at this family and found nothing to improve on the earlier post. Some additional bits: Kate (if it's the right one) was with her own family in 1891, so why Frederick was boarded out, I don't know. Perhaps her family didn't know, perhaps they didn't want to know about the little chap. It was common enough for the grandparents to bring up their grandchildren. 1911 - and a newly married Frederick Riley and wife Christina were living in Birmingham and this Frederick was a porter too. Can't find the marriage. Looking at Frederick senior again in 1911, the original image shows that he filled in the boxes about length of marriage and number of children. They had produced 8 children of whom 1 had died, so possibly there was a child before Beatrice, between Beatrice and Maud, or after Florence. The years of marriage has been heavily corrected such that it's not really possible to tell what it was, but looks like 26 corrected to 21, so would give a clue as to year of marriage and year that Kate died (in childbirth?).
Looks like one of those cases where you need each certificate before deciding the next step. If Elsie is the ancestor then perhaps get her birth certificate if you haven't already, which would tell you Kate's maiden name and therefore whether or not I am talking a load of rubbish!