Leicester square, London. We always wait till the day and go here to one of the many discount stalls to purchase our tickets but you do have to be flexible on what you see. Try not to go at the start of the school hols as I did recently. Not much in the way of discounted seats. We paid £46 for good seats for Shrek. There's no difference in the price for adults and children.
If you're thinking of going in the summer, theatre in London do something called kids week where you can go with an under 16 at very reduced prices. This years details aren't on the website yet but look here:
Enjoy! about 6 years ago