I have been to both, Whinfell was owned by Rank and then bought out by CP so doesn't have the same feel as other CP locations (we actually drive past Sherwood to go to Elevden every year)
Accommodation is more spacious on the whole at Whinfell and I do believe that the toddler pool is better and has more for little ones than Sherwood. Also as Whinfell is more out of the way and therefore less accessible to people in the Midlands or South, it felt like it doesn't get as busy.
Whinfell is more hilly though depending where you stay. As a winter holiday I would be inclined to go to Whinfell purely on the basis of more room to play in the villa.
When your children get older you can maybe travel further, we are holding out for the new CP opening in 2014! Elevden is fab because they had a fire in the 90s so they asked all previous visitors what they wanted to see when it was rebuilt, the result being that everything is in the centre, sports plaza, swimming pool, supermarket and all the restaurants bar the pub which is by the golf club I think.
We will have a 4 month old and 3 yr old when we go and just as easy to go to either. We have been to Whinfell before and 3 yr old loved the swimming complex as loads for his age there. Fancy going to a different one this year for a change but the swimming complex does not look as good or has as much for toddlers as Whinfell. Has anyone been to both and is Sherwood just as good as Whinfell or is Whinfell better?