Can I recommend Santa at Kedleston Hall near Derby (National Trust property so best to be a member) - £3 per child and you can stay for ages and do crafts, make Christmas cards etc.
Santa has a real beard and is very nice too.
Only open this w/end sat and sun 8 and 9 11am to 3pm.
Also santa is at Calke Abbey (NT again) with a tree trail we loved last year. Thi sis this w/end and next too.
Westfield is nice but make sure you don't have to queue too long.