If you are not rich you should give £100 away, really? What about the other 4 grandchildren?
'But what about the children?'. Sweet Jesus, give me strength!
Depends on your personal circumstances. If you are buying for five,and want to spend equally, I would suggest £30, if you can afford it you might like to spend more.
That said, if you were out of work and short of money then maybe less, all very personal.