Plot hole: They aren't able to travel before the birth of their children or their children become different. Despite that, he visits his father one last time before their third child is born. His Dad and he then travel back to when he was a kid, which would have changed his first 2 kids.
Suggested correction: This is not necessarily inconsistent with the movie's time travel logic. Since Tim goes back in time to visit his father and inside that particular time travel, they use time travel to go back to a day where Tim was a child. Then, they return back to the original time travel where they're playing ping pong. At that moment, both of Tim's kids were already born. So, when he returns from the ping pong scene, there's no change regarding his kids.
Also, paraphrasing the Dad, he said they aren't going to change anything. It is more like reliving a memory than changing the future to get what you want. The Dad couldn't go back to not smoke before his kids were born because that was major and would have changed the course of his life; walking a bit differently on a secluded beach that you walked on in your younger days (as long as you spent the same amount of time - presumably) would not alter the trajectory of one's life.
The slightest change in 1 second of being on the beach would absolutely affect his kids. A man produces 1500 new sperm every minute. Altering the timeline and delaying every event that is going to happen by literally 1 second would most definitely alter which exact sperm was used when conceiving both of his first two children.
Plot hole: After Tim makes the mistake of getting his sister to never have hooked-up with her boyfriend at that party in the first place, how does he undo this? There is no point where he can go back to and redo that as the point where he brings his sister back in time has already happened and you cannot undo that as technically he never brought his sister back in time in the first place, as there was no need to after she found a good relationship.
Suggested correction: I think he can as long as he is very targeted at what he's changing. They show it as 1 step, but it is a multi-step process. Step 1: travel back and never tell his sister about being able to time travel (this undoes her never dating Jimmy). BUT she hasn't learned a lesson yet. Step 2: travel back to his daughter's birthday party and not pick up Kitkat so that she has her accident. Step 3: talk to Mary and say that Kitkat needs to learn this valuable lesson, and we cannot leave until she does.
