Thishurts
Ok, I am NO expert on explain seperation to kids.
BUT Make sure you tell them TOGETHER, do not apportion blame on either you or EXDP. Make sure you tell them that you both Love them very much and will always be there for them. But you just don´t like being together anymore.
You and EXDP need to make a consious decision to not bash each other in front of the kids. Don´t talk to the kids in negative terms at all about him, and he must do the same. Thats what friends are for (ie wait untill kids are in bed and call friend to cry, scream whatever)
Both of you must be careful to not enter the one up manship league. No trying to spoil them on his days or your days. Make sure you both do fun stuff with them and make sure you both do the hard stuff. ie both work on behaviour issue if there are any. Both deal with chickenpox and colds etc. Both help with school work
Get a contact arrangement and stick to it. ie if he says he will be there are 9 on saturday he needs to be there at 9 on saturday. But be flexible with each other ie DD1 is going to this party on its on saturday so you could have her for the morning and then take her to the party and I could pick her up and bring her home etc.
I would also consider talking now whilst you can about what would happen in the future should either of you meat someone else. I think it is very difficult situation when one parents moves on quickly for the kids and the other parent to deal with.
I would suggest a party line / consistency. ie why doesn´t daddy live with us anymore. Daddy loves you both very very much, but he and mummy are much happier apart. And daddy answers questions in the same way
I am also a great fan of this line "don´t get divorced untill there is nothing more to say"
ie hash everything out between you and exdp, get all the bitterness, anger, hurt, resentment out of the way. Both speak your minds fully say it all. THEN put it behind you and work together whilst seperatly to make sure your kids are both of your prioties
Remember to keep the lines of communication with both EXDP and the kids open and I´m sure that you will get through this in time and hopefully you will all be very happy
hth, and I am sorry you are going through this.