-Steve obtains the Omnitirx at the end of Season 2. Jake and Mila aren't born until the end of season 4, but after they are born Steve quickly moves out of his parent's house before they can find out about them. With Hopper's help, Steve manages to get a halfway decent trailer out in the woods, allowing them some semblance of privacy.
-In the beginning the Omnitrix is very temperamental and will sometimes trigger without Steve's input. By the end of season 3 this stops, as the Omnitrix has fully merged with his DNA and body at this point. But until then, this causes a lot of awkward moments for Steve, especially when he's in public and only has literal seconds to hide before he transforms.
-This is how Robin finds out about Steve's situation. While working at SCOOPS during a rush hour, the Omnitrix starts beeping. Panicked, Steve tells Robin he's going on break before rushing into the back room, but she not having it, especially when there are twenty angry customers in line scowling back at her. She darts after him, ready to rip him a new one, but instead she see's him transform into a giant fish monster.
-Steve immediately tries to plead with Robin not scream, but he's having a hard time doing it considering he currently can't breath. ((The damn watch had turned him into RipJaws, an alien fish that could only breath underwater)) Still, he tries to calm her down between pained gasps, promising her that he wasn't going to hurt her, but also begging her not to cause a scene. Then, just as he's about to pass out, he feels her grab his arm, drag him over to the sink, and shove his head under the gushing faucet.
-"You so owe me an explanation fish boy!"Robin snaps at him, but she ironically sounds more concerned than scared, so Steve considers this outcome a win. By the end of the shift Robin gets the low down and to his shock and suprise, promises not to rat out him out, but in exchange he has to drive her to work.
-Steve's parents find out about Jake and Mila a year and a half after they are born. At this point the kid's are able to take on human forms thanks to the Omnitrix, so his parents have no idea they are part alien, but that does little to quell the fallout that happens next.
-Mila and Jake look older than they are, meaning his parents assume that not only had Steve been hiding these kids for at least 3-5 years, but that he had also gotten someone pregnant when he was just a kid himself. The worst part though was when Steve couldn't deny their accusations.
-Part of Steve had been hoping his parents would accepts Jake and Mila even without knowing their origin. That they would looks past the obvious questionable situation and love his kids no matter what, but that apparently wasn't the case. Seeing their outright rejection, Steve realizes he can't trust his parents any further, and just let's them assume the worse of him. They could believe whatever they wanted, as long as Jake and Mila were safe, along with their secret.
-In the end, Steve's parents disown him and leave. Steve is left with a crying Jake and Mila, and honestly Steve wants to cry with them too, but he focusing on calming them down instead. After an hour, the two kids pass out from exhaustion and Steve is alone in the bathroom, puking up his guts from stress.
-But an hour later, to Steve's astonishment, he finds Hopper and Joyce at his door looking worried beyond belief. Before Steve can even ask how they know, Joyce immediately envelopes him in a big hug, causing him to cry a little. But Steve happily returns the gesture, immensely grateful and happy to know he isn't alone in this.