Would depend on ages and specifics:
Similar aged kids, adults all have roughly same amounts of food and little alcohol - say nothing
Your kids are older and increased food consumption so your family of three eats as much as theirs of 4 - say nothing.
Similar aged kids and the adults drink double your intake of alcohol - definitely say something.
Similar aged kids and you drink/eat significantly less than other adults - say something.
They have teenagers/older kids who also increase their food bill - say something.
If there is a significant discrepancy between your families food bill, and theirs you should expect to pay proportional amounts. If there is only a small amount in it, then I wouldn't say anything for the sake of having a peaceful holiday.
I took my mum out for a meal with my sister and her family once. She said shall we split the bill halves when it came. I was really taken aback - it was her, her husband, her two teenagers and a toddler, and me and a 6 month old. She was a bit pissed off when i suggested instead that I pay for mine and my mum's meal.