My Mum does the same - although I do always remind her she also never did it with me and my sisters when we were kids :)
I tell her not to, that they have to understand it's their siblings 'day', but she says she doesn't want them to be left out. I completely understand, it's quite sweet really.
I think just keep it something simple, like a book, or a comic and a bag of sweets :-)
My parents did it for us three when we were children. I have no children but do it for my great nieces. It never arose for my niece and nephew as their birthdays are on two concurrent days (do I mean concurrent?) I do the same for my godson and his bro, they are 23 and 21 now but I still do it!
Being a twin I feel that birthdays belong to the person having the birthday (so it's your day, apart for thing 1 & 2 who obviously share their birthday). Saying that, I might change my mind if I have grandchildren.
Personally, I wouldn't - it's the birthday child's day, not the others - this is about children learning that it's not always their turn, sometimes others are in the limelight.
Short answer - don't. Birthday presents are for birthdays. Teaching the children that their turn will come is part of parenting. PS I had 6, and now have 6 grandchildren, which includes two pairs of brothers.
No I am sorry you really should stop it. I have seven grandchildren and have never done it for any of them or there mothers. They have to realise it is not there birthdays but on there special day they will have a present. Just for them. Otherwise they will always expect it. It just isnt fair on the birthday girl or boy.