I understand how difficult managing a situation like yours with the children involved is and can only mention how it's worked out in my situation. Eventually my WW and I told my two boys S10 & S8 about what was happening, to be honest she didn't have a choice as I moved out of the family home. Since that point I have done EVERYTHING I can to keep their routine the same, obviously different due to their parents no longer being together BUT their new life as stable has possible.
Their lives now are following a course that's been the same for the last month and I feel it's integrated well. I also have the first "summer" holidays coming up where we would have normally gone abroad but now are struggling to take them away for overnight stays...
I feel you wanting to allow your WS back into the FH for the holidays will take you back so far and see you throw away all the hard work you've done up to now! I really feel he needs to face up to what he's done and live the consequences of his actions, I won't tell you what to do but I would never entertain this without a concrete plan of reconciliation in place. I understand you wanting to do this for the kids but there will never be the right time to tell them what's going on so might as well be now and keep all your hard work in place in the process.
Please research into how to go about explaining your situation to you kids as this can be very difficult and quite painful.
I wish you the best, continue to be strong.
Mark.
DR'ing started March 2017
Don't blow the last bridge up from fantasy island, act "as if".