We're from the UK and only speak English, not Spanish. We had googled "Things to do in Madrid with kids" and found an article which recommended this place, amongst others. Thought the idea of a "Tooth mouse" rather than a tooth fairy was cute, so we went.
We could not get in, on the day we went, but did make a reservation for the following day. The staff did warn us that it was all in Spanish, however did say they had a guidebook in English that we could follow along with.
We had a woman leading our tour (don't know her name). Despite her speaking in Spanish and us not understanding her, she was very passionate about her job. All the Spanish kids responded well to her - they were raising their hands, shouting out answers, and enjoying themselves.
I would still recommend it, if you don't speak Spanish, because it was cheap (5 euros each), but be aware that this is the case.