Tbh, I don't think you can have an 'average' as such as there are so many different variables.
Firstly, the age of the child will affect the amount spent - their presents get a lot more expensive as they get older!
Some people will spend less but only give toys.
Other people spend more, but include essentials that the child needs to have anyway (such as slippers, pants, vests etc).
Some people buy their children toys and gifts all year round, so don't spend so much at Christmas time.
Others do not give presents except for Birthdays / Christmas, so tend to give more on those occasions.
Some people spend equal amounts on Birthdays and Christmas, whereas others I know spend far more on Birthdays and just do a few small presents for Christmas (or the other way round).
As for me, I tend to spend about £200 on my children at Christmas.
But this includes one big present of about £50, at least £50 gets spend on essentials that they would be getting anyway - they just wouldn't be wrapped up. It also includes a couple of nice outfits and then a few toys, such as DS or Wii games etc.
Also, you have to bear in mind, for Birthday they get a party so they have a max of £50 spend on presents and I do not buy them any toys / gifts outside of birthday / Christmas. If they want it outside of bday / Christmas - they have to save up for it themselves.