Isn't the first interaction in Get Together? Or something like that at least. I think you can ask other sims to flirt with or get to know other sims and i don't think that's disabled in a parent-child relationship, unless the kid is too young to date.
Also, splitting up kids after a separation or having kids be rude to a step parent aren't specific features in the game, but it's easy to do on your own. Just make some of the kids move out with one parent and the rest of them stay in manage households. And while they won't be rude to step parents autonomously, you could easily just make them do mean interactions on them.