Things are getting serious between Jennifer Garner and beau John Miller. On Oct. 16, the actress, 49, spent a cozy evening at Miller’s home with his two kids from a previous marriage, and it went so well that now they’re planning on introducing his children to her brood, Violet, 15, Seraphina, 12, and Samuel, 9. “It’s a huge deal,” a source tells Us, explaining that the pair didn’t want to involve their families “until they were 100 percent certain” that they were in it for the long haul. “Step one was meeting each other’s kids. Then they’ll look to move in together,” adds the source. “Assuming all is going well, it’ll culminate in an engagement!”…