N.,
here are my two cents in your situation. First, you need to sit down with them and tell them that although they have not seen you and daddy fighting, you guys are not 'in love' anymore. That you love each other as friends but not as husband and wife and that daddy will be moving out of the house shortly. That you guys love them no matter what and that they can see that you and daddy are good friends and get along well, but not enough to stay married to one another. That maybe in a near future daddy and you might find someone that they really love and want to marry; but that no matter what the case may be, you guys love them unconditionally and always will.
You need to prepare them, and your husband needs to put space between you guys; so when you guys are legally divorced and they see you guys with someone else, it won't be so hard for them.
I would try to get them some counseling after you speak with them, try your pastor or priest and speak to the school for your 6 year old, they have counselors that will support them through this time.
Now, I don't know how long this has been going on for, but this will be new for your girls. So I don't suggest you or your husband bring your significant other in the picture any time soon. As a matter of fact, (and I'm not judging you here) but I would not have any relationship at this point with anyone since you are still married (and that would also apply to your husband). If your girls see this, eventually and at some point they will realize that you guys were seeing other people even when you were still married.
Your (your husband and yours) significant others will stick around if they really love you, but you need to get your situation legally resolved.
Just remember that no matter how nicely you try to put it, it will be very hard for your girls, they will cry and probably have many questions; so be ready.
Be ready to love them, hug them, kiss them, and assure them that they are and always be number one in both your lives.
And the last thing I would suggest you do before you do anything I suggested is pray, pray for wisdom on how to talk to your girls and that they take it well. Believe it or not, there is a lot of peer pressure in schools about divorce, so pray girl just pray for them.
Blessings