I have two teenaged boys.......they both easily eat more than $450 a month in groceries !

That money signifies that portion of your income that you would have spent on the kids if you were still able to work......so if you live in a 3 bedroom house (instead of a 1 bedroom apartment if it were just you), groceries, utilities, clothes, out of pocket medical expenses, toys, college savings, trip to Disney......all of that is appropriate to spend 'their' money on.