Popped in to Asda (Hartlepool) for some lunchboxes - with the instructions not to get anything with cartoon pictures or anything because they're a bit too old for that now, apparently.
(Pleased at that - the merchandising ones are a lot more expensive ;) )
Found some plain blue "smart price" ones for £1.50 each - which I thought was a good deal. It was a better deal when they came up as £2 for the pair when I went through the checkout (self serve).
Apparently they're part of a "kids dine 2 for £2" deal. I have no idea what other stuff is available in that deal - as I saw nothing about it in the shop (though I didn't look).
The bags aren't very big. They'll be fine for their dinner, but you might struggle to get a drink in there, too (depending on how much you give 'em to eat/drink!)
Photo of one of the bags below (pictured next to a coke can for size comparison) together with a photo of the receipt showing the goods and the multi-buy savings.