In my experience, it almost never matters in the US. Hotels in some countries are a lot more particular. I wouldn't plan on just showing up and hoping for the best as you might find yourself trying to sort things out at the front desk or end up being told you have to pay for a larger, more expensive room. I'd recommend booking the room for 2 adults and 1 kid and then contacting the hotel to see if they can add the 2nd kid to your reservation.