If it's in your contract then you are rightly offended. You explained your reasons. I get that they had plans and were hoping to do it how they planned...BUT they signed the contract.
I don't know what else to offer up. If you don't hear back from them then I would expect that they will show up during nap. Maybe come morning if you don't hear back from them they send another text stating that since he did not reply to your first message you will assume that they plan to abide by the signed contract and you will have the child ready for pick up at 3pm.
I'm not sure if it's ok to not answer the door if the come early. Can you willfully withhold their child? Likely not. But if they do arrive during nap I would let them know that by breaking the signed contract they have nulled the contract and care is done. Can you manage without the family in your care?
Personally I would have tried to make it work (in another room with tv on and no contact with child for nap hours), if it's a one off thing I'd have gone with it...but since you stated you would not then stick with it. In this case I would expect my families to follow the contract and be respectful. Waiting an extra 30min on their part shouldn't be a big deal.