Your kids grow up too fast. This is coming from a child whose father deserted her because the other woman couldn't be bothered.
I don't think you truly appreciate how lucky you are to have an ex who will work with you on the kids. I wouldn't make any changes based on you current SO. Your kids and/or ex might resent it.
I think if worse came to worse, your current SO should give in and take the nights you all have. The kids won't even want to see you much in a couple of years.
Can you show your SO in some other way that she is very important too?