First of all, how did two kids manage to go into a love hotel even though its just to play games?
Kids, no, not at all, more like teenagers already on the upper range of that age group.
Also, the Japanese start the third year of high school when they are already 17 years old, so they become 18 years old when the third year is still ongoing, for example, Nemo has her birthday pretty close to the start of the school year, so that is why she bought that eroge (pornographic adult game) on physical format on a physical store without issue, but Mokocchi could not do it because she was still under 18 years old, Mokocchi is the opposite, her birthday is pretty close to the end of the school year.
So that means both Futaki and the guy were already 18 years old and were not using their uniforms or identified themselves using their student card, so all okey for the rules of the love hotel.