Re: Hajj and your kids.
My dad took all of us in 2004 for Hajj. We all went as a family, together, which is quite rare. That's something that I find more special, going to together and sharing this experience with your parents. If you able to, you should definitely take them before marriage.
I should add that Hajj can be costly. The onus should not be on the parents to pay for all their kids, especially if their kids are grown adults and working full-time. Kids (despite being adults) need to take some responsibility.