Children aged 2-12 can get £1.50 tickets to select morning movies at weekends and in school holidays with the Kids AM offer. What's more, accompanying adults get in for £1.50 as well.
You can buy tickets on the day, but seats can always sell out, so go early if possible. To be safe you can book online/over the phone, though you'll pay a 60p booking fee (90p for phone bookings).
Apologies if duplicate!