Here's something to consider for not just this particular topic, but for this general statement of "for kids" or other similarly weak phrase.
The person saying it is simply too ignorant to say something better.
As a professional accountant - I can provide you a strong supportable critique of why your spending habits are bad, your bookkeeping methods are bad, or your financial planning is bad.
I can't say from a position of strength whether you have a well constructed gazebo, I can only contribute superficial commentary of its aesthetic, and its ability to block the sun.
Likewise - most people can't or aren't cross genre experts of entertainment. It would take someone who is both simultaneously an avid baseball fan, and a film connoisseur, to be able to provide you an adequate statement of why baseball or film is the more worthy form of entertainment and vice versa.
Most people lack the self awareness to acknowledge their commentary on a particular topic isn't good or useful...and so we end up with reductive arguments like "for kids".
Holy shit, very well said.
As I've gotten older, I've noticed less and less of this sort of thing. During young adulthood, people are more prone to being offended by being called a child. A lot of people are insecure at this age as their peers are beginning to find work, move out of their parents house, find a companion, etcetera, and most people want to fit in and be a part of that process.
As one gets older, this desire to be more "adult-like" goes away, as without question, you are an adult, and therefore have nothing to prove. Also, as time goes by people tend to become more comfortable with themselves and may no longer be insecure about the way they are perceived by others.