A lot of TV shows aimed at toddlers aim to also be educational, to help children develop their language skills, and to help with their numeracy. They are strongly focus tested to ensure that Toddlers pay attention to the stuff on the screen. That's why there is so much repetition in shows aimed at very young children. To an adult , that might seem incredibly boring. It's entirely different for a kid. Which is why adults critiquing kids shows really need to take that kind of thing into account. It means that critics need to be able to dig into the science that goes on behind the scenes of these shows to really dig into whether they are good for kids or not.
Teletubbies was designed specifically to hold the attention of young children as young as one, and try to teach them some language skills. Therein lies its infamy. For many , sticking a one year old in front of a TV is not something we should ever be encouraging. Language development is so important, and needs the interactivity of real-life contact in order to work. In many studies, babies that age barely pay attention to television.
Which means that the actual target audience is much older developmentally than the audience it was designed for. That rendered most of its educational value useless, and turned it into a mere bauble that could distract children, but not provide them with lasting benefits. But that distraction is really useful for busy parents, and there is at least one study that suggests that out of all the TV shows you can plop your baby in front of, a Teletubbies style show will do the least damage.
But I think much of that low rating is due to the way child centred television has certain tropes, such as repetition and slow pacing , that can really turn off most critics. Having a child close at hand might not even be that helpful if they are at the wrong developmental age to get any worth out of the show. Even if they are at the right age, it's a bit of a big ask to get them to write a critical review of the work.