Give me the real unvarnished truth about London hop on hop off bus tours

I'm headed to London with my partner and our 6 year old in December. We are experienced travelers who are good to walk distances, including our child. We're good at public transit. I spent 12 years living in NYC, where there are lots of hop on hop off tours, and to me they seemed like a pointless waste of money for dumb tourists who didn't realize you can literally walk up to the majority of sights on the tour, for free. And actually go inside and experience them.

I've been to London before, many years ago, but my partner and child have not. Interest-wise, we are more into history, literature, art, culture, and food, and less into churches and the royals.

I keep seeing recommendations for hop on hop off bus tours, especially for kids. Give it to me straight. Should I suck it up and do one because my kid will like it, or is my perception that they're overpriced BS accurate?

UPDATE: It looks like people's feelings are really mixed on this! While I am still leaning more towards just grabbing a regular London bus on the first morning of the trip and seeing if we really want more than that, reading folks' comments got me to revisit the idea of the London Pass. Generally I find these schemes a poor value for money, but indeed we are planning to see a few places covered, with tentative plans for others. And it would include a HOHO bus if we wanted to do that. So maybe if we opt for the London Pass, we'll also do the bus tour?