Does your H's mum live close by. For 50/50 to work, it really requires both parents to be living very close together so that both live within easy difference of school/clubs, etc.
What causes the damage from divorce/separation is not that both parents no longer live together. Children quickly adjust to that. What causes harm is an atmosphere of hostility, a child's fear that it may be their fault in some way, significant changes to their lifestyle (e.g. moving home, far less money, losing out on friendships/clubs because of contact arrangements, a change in relationship with parents who are no longer as emotionally available either because of less physical time spent with them or because those parents are too wrapped up in their own emotions. As children get older, events like family weddings/graduations etc can cause issues. It is sadly a rare family where divorced parents can attend a family function together amicably (especially with new partners). It's not unheard of though, where a split has been conducted without animosity and with respect for all involved.
If you can be mindful of all those things and do your best to counteract them, your children will be fine. Changed yes, a little sadder about life maybe, but not damaged. And remember that any scars they bear will probably be fewer and less deep than if you had stayed together, deeply unhappy, and becoming ever more resentful/emotionally withdrawn. Few people go through life without experiencing some sort of emotional difficulties; this will be your DCs. IF you handle it well, it will be far easier for them than experiencing a completely dysfunctional family throughout their childhoods with all the messages they internalise from relationships about that.
As for telling them, I think you're right about DD probably taking it very straightforwardly, although be prepared for questions emerging over time. For DS a lot will depend on his temperament. Is he outgoing or shy? Sensitive or bombproof? Some children fare better being told in a quiet one-to-one environment where they can ask lots of questions, but IME most prefer to be told in a way that means they can avoid eye contact and not have to say anything at all at that point. To that end, a car journey where it's just the two of you can work very well. Allow time for you to stop to talk face-to-face if he wants to.
When it comes to telling him, be as matter-of-fact about it as possible. Personally, I would not say that it's your decision. IMO the trouble with making one parent responsible for the split is that it inadvertently makes a child feel like there is a protagonist and a victim. It encourages 'sides' and children often then feel emotionally responsible for the 'victim'. I think it's actually far easier for children if they feel that the decision to separate is a joint one that has been fully endorsed by both parents. It reduces the risk of them feeling any misguided responsibility and hanging on to false hope of a reconciliation. If your DS asks why, I think it's fine to say that you have been unhappy for a time but add that when you talked about it with H you both decided a split was the best course of action. I really hope your H can be on board for this.
I think it's great that you want to stress that your H is not abandoning the children by leaving. But I don't feel you need to exonerate him by blaming yourself. That's guilt talking; it's a good way of easing your conscience by assuming responsibility, but it's not a good way of helping your DC deal with things IMO. Far better to let actions speak louder than words and reassure the DC that they can still have a full relationship with both parents regardless of living arrangements - because they can. Many children actually see more of their non-residential parent and do more with them following divorce because time is scheduled and protected for them.
As long as you and your H put the DC first, you will all come through this. Good luck.