I dearly wish there were a rule book for this. It would make things so much simpler, wouldn’t it? Unfortunately, there isn’t such a thing. How to introduce a new partner – and when – is dependent on many things: how long you and your husband lived apart, the personalities of the children, the level of civility between you and your ex, and the ability of the new guy to take a back seat to the needs of kids for awhile are all factors that need to be considered. Because it is so complicated, I generally advise parents of young kids to take it slow. If your kids have a good relationship (even a ‘pretty good” relationship) with their father, they aren’t going to welcome a “replacement dad.” They need time to acclimate to going back and forth between two homes and to being without one or the other of their parents each day. Meanwhile, it falls on you and your boyfriend to remember that the kids’ needs come first. You may be ready to move on but the kids may have some strong negative feelings toward a live-in guy who takes mom’s time and attention.
Ideally, I’d suggest that the boyfriend live elsewhere for awhile and make visits to you and the kids, gradually extending the length of the visits. This gives him and the kids time to get used to each other. It also gives you the opportunity when he leaves to focus entirely on the kids’ needs. You and your new guy will have all weekend every weekend to be lovebirds. Yes, I know that what I’m proposing is more expensive. Maintaining 2 apartments is more costly than having your boyfriend move in. But I suggest to you that it’s a worthwhile investment in making sure that he and the kids make a good adjustment to each other. Further, on the off chance that things don’t work out between you, it protects your kids from experiencing yet another loss of a parent figure.
Those are all general comments that may or may not apply to your children. It might be helpful for you to go to a family therapist for a few sessions to get some specific input about how your kids are doing and what you can do to help them – and you – with the transition.
I wish you well.