I agree, my parents told us very plainly and clearly that they were separating, that they did not love us any less etc etc.
I am also a step-parent. I have had my 2 SDDs for 7 years now and the youngest struggled more than the older I think. You know what she was most confused over? As she never saw or heard her parents arguing and they seemed good friends (which they were and still are it is just that the marriage was over) she could not work out why they could not just continue to live together. This is natural, kids are unable to understand the depths of love, hell, I don't think many of us adults do sometimes either.
SO, what to watch out for. Well, kids are very literal, so if you say they are going to spend time with you or with their dad then is HAS TO HAPPEN. No letting down at the last minute, no sudden change of plans, even if the plans means a trip to some exciting place if they were expecting to see a particular parent then it has to happen.
SDD2 has issues with emotion etc and struggles to express empathy or say how she feels/felt. We found that putting a big chart on her wall and filling in each weekend, where she would be, who she would be seeing etc really helped. She then felt more in control and could look forward not backwards.
i know you wouldn't do this but NEVER EVER slag the other partner off or case any blame either way. Kids will struggle to think of you are being 'nasty' to the other parent so never even a passing comment. Make sure all your phone calls to friends/relatives are once they are in bed or where they cannot hear you. A one sided conversation can be misinterpreted, especially if you are upset or angry.
Be prepared for LOTS of questions, be calm and patient and offer a reasonable explanation even if you are saving them from the whole truth sometimes. A little white fib doesn't hurt, just make sure you are consistent and keep them at a level of loving and nurturing. Don't deal in lies, it never works, however, they don't need to know the exact ins and outs of 'adult' issues whilst they are still so small.
NEVER let them do something then say 'don't tell your Dad' otherwise they will play it against you at some time (!) and be prepared for them to play one of you off against the other without allowing yourself to feel hurt. Like, 'At Daddy's we can stay up till 10pm and eat chocolate'. Obviously not true and a quick text out of their earshot can give you a double check. Plan your rules between you and DH and be consistent at both houses.
Never carry a punishment over from one house to the next, they need to know you are not 'dobbing' them in to the other parent. If they are due a punsishment for something they have done with you then carry it out at your house in your time allowing their time with the other parent to be all about that relationship.
SOrry, I have waffled on and on! I am sorry to hear that you have reached this point but you are being kinder to the kids to do it now rather than when you are at your wits end. You cannot be the parent they need if you are stressed and upset. Good luck x