This is a possible downside to “letting things slide.” Now you have unhappy guests who are dishonest to book. I agree that you shouldn’t worry about how to respond yet, perhaps they won’t review or will leave you a good review.
You have described the listing as an apartment at lower level and/or as being downstairs. It also accommodates at least 5 people plus a toddler? It sounds like a separate, self contained unit. In that case it’s not unreasonable for guests to expect to be able to have some control over their temperature. Having a reasonable room temperature should be covered by the cost you are charging the guest. Within reason this temperature should be determined by the guest, not the host.
Keep in mind that the guest gets to leave a review and especially when you are just starting a bad review can take a long time to recover from. Airbnb is raising the standards they expect hosts to maintain as well. So you have to find a balance between dictating to the guest what temperature they will get in the rental and their comfort.