I agree with @ BigBreakfast too many people here will tell you that you should leave. But IMO too many people are too quick to walk away from a relationship and throw their children into an unsettled life in pursuit of their own perceived happiness which may never actually happen.
I think that once you have children you owe it to them to keep their own lives as stable as possible, assuming there is no actual abuse or violence (and people will try to paint his not always helping with the kids as abuse, so watch out for that).
The grass isn’t always greener, and even for the kids it will be massively unsettling, anyone who tells you that the children are better off is generally saying that to justify their own decisions.
But think about:
Not seeing your children for 50% of the time.
Having your children potentially favour being with their dad over you.
Not ever meeting someone else because you can’t find someone your age who would want to be with a woman with two kids, while your DP moves on and brings a new woman, and potentially more siblings, into your children’s lives...
Your children seeing another woman as a mother figure (as they’re young this is very possible, likely even), wanting to see their dad more because their baby brother/sister is there.
Your children being upset because mummy and daddy can’t both go to their school play.
Not being able to have all their toys in one place but having to choose what they can play with based on which house they’re in.
These are obviously worst case scenarios, but is your relationship with your DP really bad enough to want to do all that?
You need to talk to him. If you’re unhappy then maybe he is as well? How would you feel if he was the one who walked out tomorrow?