When one gets invited somewhere, is it true they aren't expected to pay?

It depends. It is rude to invite someone if you know they can't afford it. And since you are kids I'm sure his parents would definately pay for you if they like you. My guess is your parents have had to work hard for their money and can't afford to throw it around so they wouldn't expect or want someone to spend money on you guys since you can't return the favor.