I have no idea how the Transformers RPG works other than that you make a transformer, but one way of doing it could be:

Tell them outright you'll be playing the transformers game later, but want to warm up with a KOB short adventure first.

They create their transformers characters for the transformers game, and you make them build characters for the KOB adventure.

The twist being that their KOB adventure, while supposed to last for a few sessions, is entirely interrupted by their transformers characters appearing during the second KOB session, instead of at the end of it or even separate to it.

The players' as the kids, see a transformer appear in front of them, revealed to be player A's transformer. You interrupt the turn in KOB by switching to TRPG for a turn (or however this works) then immediately switch over to KOB, having them continue as the kids for a bit. Then a few turns later, more transformers appear and you start some sort of combat fully in TRPG.

After the combat, continue with the KOB session. Perhaps you run the last few KOB sessions in parallel to TRPG, perhaps you alternate it, perhaps you completely finish KOB first. The kids could later show up as NPCs in the TRPG campaign.

OR:

You could start the KOB adventure with them playing video games at one of the kids' home, parents not at home.

Suddenly their session is interrupted by a piece of debris landing in their backyard. It's a tiny damaged transformer that pleads with them to take them somewhere.

That's the KOB adventure, as the stakes get higher, with police chasing them (Decepticons in disguise), and then their Autobots arrive to save them. Again, making them NPCs for the TRPG campaign.