I have always heard that visitation, etc. that is exercised in the interim can, if it goes on long enough, be seen as a “standard” once the case comes up. Example, if you only see your children 2-3 days a week in the time between now and your case, that this could be seen as the best thing for the children; since it’s gone on for so long and it is presumably what they are used to. Of course, if it’s only 2-3 days a week because she’s refusing you from having them any more than that, then be prepared to produce proof via written documentation. Then again, if you only want them 2-3 days a week and you’re fine with that going forward, then there’s no problem.
You have to do what you think is best, but there is absolutely no way I would not see my kids for weeks or months at a time just because my ex wants them 100% of the time. You don’t want to go up to their school and start a scene of course if she’s there when you get there to pick them up; or start a scene in front of your children anywhere for that matter. But, and this is just me, if I were in a situation where my ex was refusing to let me keep my children for any overnights at all, and I had the chance to pick them up from school and take them home a few nights a week or over a weekend to spend some time together, I would certainly do that. If she then starts doing stuff in the future to prevent you from taking them for visitation (like picking them up from school early, not taking them to school, contacting the school to try and prevent you from picking them up, etc.), then that would certainly not look good for her going forward once your case comes before a judge. Document anything like this when it happens (while the dates, times, and circumstances are stil fresh in your mind).
Of course, if she has something coming up on a given weekend from time to time that she wants the children to attend with her, it’s also in your best interest to work with her; perhaps agree to swap weekends- you let her take them that weekend, and you get the children on her next weekend. You also want to show that you are flexible and willing to compromise when faced with inevitably changing schedules and activities, etc. Again, document, document, document!
Whatever you do, and this cannot be overstated, document absolutely everything. If she pulls them out of school early because she’s afraid you’ll pick them up, ask for a copy of the sign-out sheet from the office (most schools, if not all, require you to sign your kids out if it’s before the end of the school day) and note that you were supposed to pick them up from school that day and she came and signed them out early. If she refuses to let you have them for your weekend, write that down (date and time of conversation), record every conversation with your ex if possible. If you pick your children up from school to stay with you over the weekend because of her continued refusals to agree to the children having overnights with you, document that and WHY you did it (i.e. "she refused to let me have them after having them herself for xx days prior - in effect changing our previously agreed upon visitation schedule).
Again, best of luck to you. I’ve been down a bit of this road myself.