First, setting some baselines. The American Academy of Pediatrics recommends limited, but not zero, screen time for most children above 2. Under 2, and in particular under 18 months, no screen time other than video chat (Facetime/Skype/etc.).
For children younger than 18 months, avoid use of screen media other than video-chatting. Parents of children 18 to 24 months of age who want to introduce digital media should choose high-quality programming, and watch it with their children to help them understand what they're seeing.
For children over two, up to about 5, an hour a day is reasonable, so long as it is high
quality programming. (What you describe would qualify as high-quality, I believe).
For children ages 2 to 5 years, limit screen use to 1 hour per day of high-quality programs. Parents should co-view media with children to help them understand what they are seeing and apply it to the world around them.
For children over six, there is no longer a blanket recommendation; the recommendation is to simply ensure screen time is not taking the place of other needs.
For children ages 6 and older, place consistent limits on the time spent using media, and the types of media, and make sure media does not take the place of adequate sleep, physical activity and other behaviors essential to health.
Many reasons exist to suggest limiting screen time. Adverse associations with sleep, attention spans, obesity, and language development in particular are called out.
Going into these in a bit more detail:
Obesity. Some of the connection with obesity is likely due to the connection between obesity and sleep. Eating during screen time is another. Advertising (and the connected social pressure) is also a related issue, though less so for non-television (or non-ad-based-tv) screen time of course. The linked study of studies above goes into more detail on this connection; it also emphasizes that screen time can sometimes be beneficial.
Sleep. The link between screen time and reduced sleep seems strong, but it's been very difficult to find a causal link. Theorized links include simply spending more time on screen that could have been spent on sleep; light from screens causing reductions in natural sleepiness; and decreases in physical activity relating to children not spending time outside as much. Of course, these are all pretty hard to test.
Attention spans. Very young children (<2 or <3) seem to have some correlation between reduced attention spans later in life and some kinds of programming. It's become more clear though that specifically educational programming likely does not have this connection. The linked study finds that below 3, non-educational programming is linked to reduced attention spans 5 years later, while at 4 or 5 years it is no longer linked to reduced attention spans later in life. It's not clear why this is present only at 3 or earlier.
Language development. Again, really only an issue very early on. Theorized reasons are largely based on the fact that younger children develop language from listening to adults speak (to and to each other), and screen time often replaces these interactions. Much of this language development happens in the first two years of life.
Beyond the research, which mostly focuses on younger children, the key is consistency and ensuring it does not interfere with other activities - in particular, entirely replacing active play. In these older children, obesity and sleep are still concerns, at least potential concerns. Ensuring children are not using screens to the exclusion of sleep, homework, chores, or active play is critical, and of course ensuring children develop their own internal controls beyond parental controls.
