Family-friendly Hotels in Plymouth

We visited Plymouth for a wedding. I booked a room for myself and when I went back later to double check the room types available, it gave me a crazy low rate. I figured I would be a fool not to book it, so I booked another room for my parents. I then received my confirmation email showing the low rate. I figured if we checked in and the hotel wouldn't honor the rate, we could share a room since we both had 2 beds. We received a text to check in the morning of arrival which confirmed our rate of a total of $100 for 2 nights. When we checked in at the hotel, we were given our keys and there was no mention in a change of rate. However, when we checked out, we were emailed a receipt showing $768 and my credit card was charged this rate, a whopping $668 more than agreed upon. I talked to the front desk and was told to contact the front desk manager, who was not there at the time. I have since emailed him several times and left a voicemail, but I have not gotten a response. I think it is wrong not to honor the rate agreed upon and I don't see how you can change the rate at check out. It is one thing to change a rate prior to or at check in so that the guest can decide not to keep the reservation, but not at check out when the guest is left with no options. It isn't my fault that your booking platform made the mistake. The hotel itself was just ok. My room was alright with a nice shower and I didn't have any issues with it. However, several family members had problems with (lack of) cleanliness with their rooms. They found objects in their rooms from previous guests, including dirty socks, a pacifier and someone else's credit card. The pool and splash pad were nice though we were disappointed that the hot tub was out of order. The location is great but if staying in the area again, I will stay somewhere that honors the booked rate.