1 big pot maybe... but each have their own budget.
Think of it as a dad takes his kids shopping and has £30 to spend on them. So £10 each
Child "I" picks a book for £5
Child "P" picks a doll for 12
Child "KL" picks a dvd for £10
When he gets to the cash desk he has enough money. But "I" wants to purchase something else from shop next door for the rest of their £5. So what does he do... The only logical answer is for "P" to pick something that costs their share so that "I" can goto the other shop.