I think part of that thing is many children, especially younger ones, likely can't read to a standard where they could follow subtitles. That's why kids shows tend to be dubbed even in countries that usually subtitle.
I can see subtitles as being problematic for programmes intended for younger children, but it's less of a problem for older children and young teenagers. There is a flipside of (properly done) subtitles in that it might help improve children's reading ability.
It really is true that millions of children worldwide have to either learn another language or read subtitles in order to watch a cartoon! Dubbing is increasing in use but is still mainly used by TV channels / producers with deeper pockets or in languages with large numbers of speakers. There are generally more economic benefits of dubbing TV programmes in Spanish or Arabic than they are in Albanian or Balochi.
Another factor that can't be ignored is an increase in the number of children in Britain who know a language other than English, although the number and diversity of languages in use is challenging from an economic perspective, including the provision of TV programmes.