Beth to be honest they advertise their free child places but
they don't tell you the dates. I know they are normally out of season or off peak and I think most tend to be in june and september.I have never seen any for the xmas and new year period.There will be someone on here who will be more accurate.Good luck with your search
Not sure about Virgin but most other airlines tend to have the free child places for off peak(not school holidays). We got a free child place with airtours last year for this years trip. But I'm still not sure how we managed it as I know my friend for another holiday got the free child place as she flew from Newcastle instead of glasgow. It may just be sheer luck but good luck in trying.