Hi Nieve
The difference between FindMyPast and Ancestry for this type of search is that for FindMyPast, you will have to view each individual transcription to see whether the person has children of an age that excludes your Ellen, whilst on Ancestry, you can just hover over where it shows 1891 census (for instance) and you will be able to see the transcription without having to open it. That will save a lot of time, if you are searching through 100s of listings.
Happy Christmas and good hunting.
Lizzie
ps. Overall, I prefer FindMyPast but subscribe to both as each has its plus points.