I think the difference is the same as a G-rated and a PG-rated movie.Sure,the more audiences you have,the better,but the G-rating indicates that it's too exclusively targeted to little kids,while a PG-rating may be a sign that audiences other than kids may enjoy them and may want to consume their products as well,not just for the little kids.That's why some movies throw a "damn" or even "s***" just to avoid the G-rating(which was deliberately done to the Secret of NIMH,for example,and yet it was still rated G,despite even having blood.). The equivalent of that in video games would be the EC rating,usually for games considered educational and for preschool kids.While the E rating is a game that may be harmless,but with content that literally everyone can enjoy(like a Super Mario or a Pokemon game),there's also a E+10 rating for games with content just a bit more intense content than a normal "E"
I hate when people use "kiddie" as a reason to put down a game. If they want to say that the game doesn't appeal to them and not their type of game I can understand. To call it "kiddie" though just makes that person sound like an insecure child that wants to appear more grown up.
Games that can actually be even considered "kiddie" are ones that are literally geared towards kids ages 5 and under. Things like Barney, Dora The Explorer, Go Diego Go etc. Whereas games like Mario, Donkey Kong etc. can be enjoyed by anyone regardless of age. Just like a lot of Disney and Pixar movies.