How to make my characters detectives?

Now, it's already a pretty light story. It's about about a Kid Detective and his friends who pilot a Mini-Mecha to pose as an adult. But I want them in the story to be a bit more...official than, say, Mystery Incorporated.

I do have a handwave for why they're commissioned by the police: the chief, having been a master detective herself and bored after having been Kicked Upstairs, has no problem seeing through the ruse, and is playing along to amuse herself.

But I can't get the sequence of events straight in my head. They should probably be private for at least their first case, but I'm not sure exactly how they go about getting a full police commission (or if that's even the correct term), as in retrospect I haven't actually seen this done in fiction, and my attempts to research the process have yet to turn up anything. I don't exactly need a strict adherence to realism, but I'm not sure who should be contacting who, what all is involved, etc.

I have no idea, but to make things easier for anyone else who may wish to contribute, it would help if you'd tell us the time period and location of your story.

Ah, sorry. Modern day, possibly 20 Minutes into the Future. Implied to be an analogue of the United States with similar laws.

Technically speaking, it's a World of Funny Animals to give the characters' Mini-Mecha something to fight. A human can only bulk up so much until you have to claim it's a cyborg or a mutant. Nobody's going to bat an eye though if a seven-foot-tall Top-Heavy Guy robot dragon is getting punched around by a bipedal rhinoceros.

If it's a future America, have the law be heavily privatized and have the kids purchase an inspector's license with their piggy bank savings.

See, because it's a piggy bank and they want to be cops...

Hmm, interesting idea, actually. I could just play around with how the law works like in Phoenix Wright. It would also explain the sheer amount of excessive force that's allowed whenever they get into a fight.