Re comments by 'arethereanyleftatall'
User1484 - regarding your 'confusion' as to why you wouldnt buy 2 presents. Maybe they can't afford to buy 2 presents?
Anyone can afford 2 presents (for 2 children.) Just take what you would have spent on one (say ten or twelve quid,) and split in two. Ergo, get them something for £5-6 each. There are tons of things you can get for this amount from many shops including home bargains, b & m, primark, tesco, asda, the works, and so on... and even the entertainer and smyths sell stuff for around this amount too.
For around £5-6, you can get a jigsaw, a paint-by-numbers, a colouring book/puzzle book and pack of pens, a small beanie, a small doll, a personalised mug, a box of roses or quality street, a soft toy, slippers, socks, a t-shirt, a friendship bracelet or bead making kit, earrings or some kind of jewellery from 'Claires,' a child's novel, a poster or photo of their favourite video game star or pop star, (or a keyring or mug with said star's face on,) etc etc. There are loads of things you can get for girls AND boys for £5-6.
As I said, if I knew both children, (and particularly if they were siblings as in the OP,) no WAY would I only be buying for the one. That's just mean IMO.