Gamera saves Toshio from falling off the lighthouse in the original film and this is used as evidence that Gamera is a "good turtle". No one involved mentions that Toshio wouldn't have needed saving if Gamera hadn't smashed the lighthouse in the first place. You don't get to be a hero just by rescuing someone from peril that was entirely your fault to begin with.
It was really a shallowly-explored plot point. It would have been fascinating to explore why Gamera makes an exception for children, but instead they just presented it as a draw for children.
Gamera is an Anti-Hero. Sure, he'll save children from harm, but he's still going to smash buildings and whatnot. Plus, you forget that the original Gamera was trying to cash in on the success of the Godzilla films. Giant monsters rampaging across Japan is pretty standared, regardless of whether or not they're good or bad. The "Friend to All Children" aspect of Gamera was created to make him more kid-friendly to young audiences. Remember, at the time (mid 1960s), the Godzilla films were becoming more light-hearted and geared towards kids due to more and more children going to see the Godzilla films at the time. Kadokawa Daiei Studios wanted to cash in on this, so they created Gamera.
In the original film, it didn't appear to me that Gamera was the friend to all children. It appeared that he accidentally saved Toshio by mistake, and it was Gamera that endangered him in the first place. It also seemed like Toshio had a very unhealthy obsession with Gamera to the point where he nearly died in a train crash just to see him. It wasn't until Gamera vs. Gyaos that Gamera was flanderized into the Friend Of All Childen, and in all the films I saw after the first one, Gamera usually fights the monsters as far away as possible from any humans nearby. Even by Zigra the adults are happy to see him when he arrives to put out fires (aka consume them). The only instances where Gamera does attack humans are the original, the opening of Barugon, Viras and Super Monster (the last two are due to him being Brainwashed and Crazy and using Stock Footage from the first film.)
Why did the military decide to make Gamera a priority in the third Heisei film? I get it, he caused a lot of death and destruction killing the Gyaos...but the Gyaos are confirmed to be capable of wiping out an entire civilization that was more advanced than humanity. Then Iris, a known maneater who's actively hostile towards humanity, shows up...and they decide to fire on Gamera while he's in the midst of fighting Iris, who was heading directly for a populated area. Are the military suicidal? I'd understand if they'd played up the idea that Iris' cult was responsible for this, but I think I missed that if they said anything about that.