When did it begin to make movies out of children's shows?

[Hawkee]

In the movie industry I noticed that children's shows had movies based on them and I have never figured it out? Is it because children's shows are simple to make movies out of or is it to attract the younger audience? Because ever since Sesame Street debuted Follow That Bird in 1985 this truly started the fad of movies based on children's shows and Sesame Street repeated it with their second movie Elmo In Grouchland and it also became a hit. Another children's show to do this was Barney And Friends when the film Barney's Great Adventure debuted in theaters in 1998 and it surprises me that movies based on children's shows are hits. The same thing happened with Thomas And The Magic Railroad which was based on Shining Time Station and was an okay hit. But I think as time goes by we will continue to see more children's shows become movies in the future ahead
Bestie